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Diminished data rate with logic output optoisolator


Shift Register output to RelaysSimple switch with optoisolator - CTR relatedOptoisolator with Logic Output - How To Use IC Circuit Example?OptoIsolator Reliability?connect a triac to optoisolatorHelp with using 4N25 optoisolatorOptoisolator Clarification - Logic Output vs. Transistor/Photovoltaic Output?Replace optoisolator with a resistor






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;








4












$begingroup$


I am using a logic output type optoisolator (H11L1S) that has a nominal data rate of 1 MHz, yet in practice I can't even achieve 100 kHz. Where am I going wrong? Is this maximum data rate unattainable?



Here is the relevant circuitry:



Schematic



I am driving the LED at 2.8 mA, which is well above the minimum turn-on current of 1.6 mA (plus 10 % guard band suggested by the datasheet). Below is a scope capture of the clock signal (ADC_SCK, yellow) and LED cathode (blue). Once the transistor turns off the cathode voltage takes more than $5mu s$ to reach +3V3 -- i.e. the LED turns off very slowly -- such that the receiver does not register the change in state.



Clock, opto signals



This means the hot-side circuitry (ADC_SCLK, blue) sees a very slow clock:



Received clock










share|improve this question









$endgroup$




















    4












    $begingroup$


    I am using a logic output type optoisolator (H11L1S) that has a nominal data rate of 1 MHz, yet in practice I can't even achieve 100 kHz. Where am I going wrong? Is this maximum data rate unattainable?



    Here is the relevant circuitry:



    Schematic



    I am driving the LED at 2.8 mA, which is well above the minimum turn-on current of 1.6 mA (plus 10 % guard band suggested by the datasheet). Below is a scope capture of the clock signal (ADC_SCK, yellow) and LED cathode (blue). Once the transistor turns off the cathode voltage takes more than $5mu s$ to reach +3V3 -- i.e. the LED turns off very slowly -- such that the receiver does not register the change in state.



    Clock, opto signals



    This means the hot-side circuitry (ADC_SCLK, blue) sees a very slow clock:



    Received clock










    share|improve this question









    $endgroup$
















      4












      4








      4





      $begingroup$


      I am using a logic output type optoisolator (H11L1S) that has a nominal data rate of 1 MHz, yet in practice I can't even achieve 100 kHz. Where am I going wrong? Is this maximum data rate unattainable?



      Here is the relevant circuitry:



      Schematic



      I am driving the LED at 2.8 mA, which is well above the minimum turn-on current of 1.6 mA (plus 10 % guard band suggested by the datasheet). Below is a scope capture of the clock signal (ADC_SCK, yellow) and LED cathode (blue). Once the transistor turns off the cathode voltage takes more than $5mu s$ to reach +3V3 -- i.e. the LED turns off very slowly -- such that the receiver does not register the change in state.



      Clock, opto signals



      This means the hot-side circuitry (ADC_SCLK, blue) sees a very slow clock:



      Received clock










      share|improve this question









      $endgroup$




      I am using a logic output type optoisolator (H11L1S) that has a nominal data rate of 1 MHz, yet in practice I can't even achieve 100 kHz. Where am I going wrong? Is this maximum data rate unattainable?



      Here is the relevant circuitry:



      Schematic



      I am driving the LED at 2.8 mA, which is well above the minimum turn-on current of 1.6 mA (plus 10 % guard band suggested by the datasheet). Below is a scope capture of the clock signal (ADC_SCK, yellow) and LED cathode (blue). Once the transistor turns off the cathode voltage takes more than $5mu s$ to reach +3V3 -- i.e. the LED turns off very slowly -- such that the receiver does not register the change in state.



      Clock, opto signals



      This means the hot-side circuitry (ADC_SCLK, blue) sees a very slow clock:



      Received clock







      opto-isolator






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 8 hours ago









      calcium3000calcium3000

      1,5931 gold badge6 silver badges24 bronze badges




      1,5931 gold badge6 silver badges24 bronze badges























          3 Answers
          3






          active

          oldest

          votes


















          4














          $begingroup$

          Take another look at the datasheet, specifically at the 'recommended' RL pull-up resistor value.

          That's 270 Ohms, while you're using 15k.



          That device sources very little (if any) current when the output goes high, so the rise time you're seeing is directly proportional to that RL pullup resistor you're using (combined with the gate capacitance of your Q40 and any parasitics).






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
            $endgroup$
            – calcium3000
            8 hours ago


















          0














          $begingroup$

          The FET can pull up but it can't pull down. There is only the 15k resistor to pull down. It's the falling edge at R187 that is slow.






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
            $endgroup$
            – calcium3000
            7 hours ago


















          0














          $begingroup$

          To speed up the opto's input, two things can be done.



          1. Decrease R189 to 150Ω to supply ~10mA to the LED when ADC_SCK is active. (3.3v - 1.15v)/10mA = 143.3Ω

          2. Add a small "speed-up" capacitance across R189. For Z=50Ω and R189=150Ω, Xc should be:

          $150Ω || X_C = 50Ω$



          $frac1150Ω + frac1X_C = frac150Ω$



          $0.00overline 6 + frac1X_C = 0.02$



          $0.02 - 0.00overline 6 = 0.01overline 3$



          $frac10.01overline 3 = 75Ω$



          $ X_C = frac12pi fC$ , pluging in 1MHz for $f$ ,



          $ 75Ω = frac12pi cdot 1Mcdot C$ and solving for C:



          $ 75Ωcdot 2picdot 1M = frac1C$



          $ 471,238,898.038 = frac1C$



          $C approx 2.2$nF



          You can also add a small speed-up cap across the unlabeled resistor on Q18-A's base. However, note in the datasheet that it specifies the maxiumum $t_on$ and $t_off$ of 4µs. $frac14µs$ = 250kHz, not 1MHz!






          share|improve this answer











          $endgroup$














          • $begingroup$
            Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
            $endgroup$
            – calcium3000
            7 hours ago













          Your Answer






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          3 Answers
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          3 Answers
          3






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          4














          $begingroup$

          Take another look at the datasheet, specifically at the 'recommended' RL pull-up resistor value.

          That's 270 Ohms, while you're using 15k.



          That device sources very little (if any) current when the output goes high, so the rise time you're seeing is directly proportional to that RL pullup resistor you're using (combined with the gate capacitance of your Q40 and any parasitics).






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
            $endgroup$
            – calcium3000
            8 hours ago















          4














          $begingroup$

          Take another look at the datasheet, specifically at the 'recommended' RL pull-up resistor value.

          That's 270 Ohms, while you're using 15k.



          That device sources very little (if any) current when the output goes high, so the rise time you're seeing is directly proportional to that RL pullup resistor you're using (combined with the gate capacitance of your Q40 and any parasitics).






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
            $endgroup$
            – calcium3000
            8 hours ago













          4














          4










          4







          $begingroup$

          Take another look at the datasheet, specifically at the 'recommended' RL pull-up resistor value.

          That's 270 Ohms, while you're using 15k.



          That device sources very little (if any) current when the output goes high, so the rise time you're seeing is directly proportional to that RL pullup resistor you're using (combined with the gate capacitance of your Q40 and any parasitics).






          share|improve this answer









          $endgroup$



          Take another look at the datasheet, specifically at the 'recommended' RL pull-up resistor value.

          That's 270 Ohms, while you're using 15k.



          That device sources very little (if any) current when the output goes high, so the rise time you're seeing is directly proportional to that RL pullup resistor you're using (combined with the gate capacitance of your Q40 and any parasitics).







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 8 hours ago









          brhansbrhans

          10.3k2 gold badges25 silver badges32 bronze badges




          10.3k2 gold badges25 silver badges32 bronze badges














          • $begingroup$
            Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
            $endgroup$
            – calcium3000
            8 hours ago
















          • $begingroup$
            Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
            $endgroup$
            – calcium3000
            8 hours ago















          $begingroup$
          Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
          $endgroup$
          – calcium3000
          8 hours ago




          $begingroup$
          Ah, good call. But that shouldn't affect the slow LED fall times, right? Do I need a push-pull circuit there?
          $endgroup$
          – calcium3000
          8 hours ago













          0














          $begingroup$

          The FET can pull up but it can't pull down. There is only the 15k resistor to pull down. It's the falling edge at R187 that is slow.






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
            $endgroup$
            – calcium3000
            7 hours ago















          0














          $begingroup$

          The FET can pull up but it can't pull down. There is only the 15k resistor to pull down. It's the falling edge at R187 that is slow.






          share|improve this answer









          $endgroup$














          • $begingroup$
            Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
            $endgroup$
            – calcium3000
            7 hours ago













          0














          0










          0







          $begingroup$

          The FET can pull up but it can't pull down. There is only the 15k resistor to pull down. It's the falling edge at R187 that is slow.






          share|improve this answer









          $endgroup$



          The FET can pull up but it can't pull down. There is only the 15k resistor to pull down. It's the falling edge at R187 that is slow.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 7 hours ago









          JustmeJustme

          7,6922 gold badges8 silver badges21 bronze badges




          7,6922 gold badges8 silver badges21 bronze badges














          • $begingroup$
            Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
            $endgroup$
            – calcium3000
            7 hours ago
















          • $begingroup$
            Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
            $endgroup$
            – calcium3000
            7 hours ago















          $begingroup$
          Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
          $endgroup$
          – calcium3000
          7 hours ago




          $begingroup$
          Ah, so a heavier load on the P-FET is part of the problem, eh? I'll give that a shot.
          $endgroup$
          – calcium3000
          7 hours ago











          0














          $begingroup$

          To speed up the opto's input, two things can be done.



          1. Decrease R189 to 150Ω to supply ~10mA to the LED when ADC_SCK is active. (3.3v - 1.15v)/10mA = 143.3Ω

          2. Add a small "speed-up" capacitance across R189. For Z=50Ω and R189=150Ω, Xc should be:

          $150Ω || X_C = 50Ω$



          $frac1150Ω + frac1X_C = frac150Ω$



          $0.00overline 6 + frac1X_C = 0.02$



          $0.02 - 0.00overline 6 = 0.01overline 3$



          $frac10.01overline 3 = 75Ω$



          $ X_C = frac12pi fC$ , pluging in 1MHz for $f$ ,



          $ 75Ω = frac12pi cdot 1Mcdot C$ and solving for C:



          $ 75Ωcdot 2picdot 1M = frac1C$



          $ 471,238,898.038 = frac1C$



          $C approx 2.2$nF



          You can also add a small speed-up cap across the unlabeled resistor on Q18-A's base. However, note in the datasheet that it specifies the maxiumum $t_on$ and $t_off$ of 4µs. $frac14µs$ = 250kHz, not 1MHz!






          share|improve this answer











          $endgroup$














          • $begingroup$
            Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
            $endgroup$
            – calcium3000
            7 hours ago















          0














          $begingroup$

          To speed up the opto's input, two things can be done.



          1. Decrease R189 to 150Ω to supply ~10mA to the LED when ADC_SCK is active. (3.3v - 1.15v)/10mA = 143.3Ω

          2. Add a small "speed-up" capacitance across R189. For Z=50Ω and R189=150Ω, Xc should be:

          $150Ω || X_C = 50Ω$



          $frac1150Ω + frac1X_C = frac150Ω$



          $0.00overline 6 + frac1X_C = 0.02$



          $0.02 - 0.00overline 6 = 0.01overline 3$



          $frac10.01overline 3 = 75Ω$



          $ X_C = frac12pi fC$ , pluging in 1MHz for $f$ ,



          $ 75Ω = frac12pi cdot 1Mcdot C$ and solving for C:



          $ 75Ωcdot 2picdot 1M = frac1C$



          $ 471,238,898.038 = frac1C$



          $C approx 2.2$nF



          You can also add a small speed-up cap across the unlabeled resistor on Q18-A's base. However, note in the datasheet that it specifies the maxiumum $t_on$ and $t_off$ of 4µs. $frac14µs$ = 250kHz, not 1MHz!






          share|improve this answer











          $endgroup$














          • $begingroup$
            Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
            $endgroup$
            – calcium3000
            7 hours ago













          0














          0










          0







          $begingroup$

          To speed up the opto's input, two things can be done.



          1. Decrease R189 to 150Ω to supply ~10mA to the LED when ADC_SCK is active. (3.3v - 1.15v)/10mA = 143.3Ω

          2. Add a small "speed-up" capacitance across R189. For Z=50Ω and R189=150Ω, Xc should be:

          $150Ω || X_C = 50Ω$



          $frac1150Ω + frac1X_C = frac150Ω$



          $0.00overline 6 + frac1X_C = 0.02$



          $0.02 - 0.00overline 6 = 0.01overline 3$



          $frac10.01overline 3 = 75Ω$



          $ X_C = frac12pi fC$ , pluging in 1MHz for $f$ ,



          $ 75Ω = frac12pi cdot 1Mcdot C$ and solving for C:



          $ 75Ωcdot 2picdot 1M = frac1C$



          $ 471,238,898.038 = frac1C$



          $C approx 2.2$nF



          You can also add a small speed-up cap across the unlabeled resistor on Q18-A's base. However, note in the datasheet that it specifies the maxiumum $t_on$ and $t_off$ of 4µs. $frac14µs$ = 250kHz, not 1MHz!






          share|improve this answer











          $endgroup$



          To speed up the opto's input, two things can be done.



          1. Decrease R189 to 150Ω to supply ~10mA to the LED when ADC_SCK is active. (3.3v - 1.15v)/10mA = 143.3Ω

          2. Add a small "speed-up" capacitance across R189. For Z=50Ω and R189=150Ω, Xc should be:

          $150Ω || X_C = 50Ω$



          $frac1150Ω + frac1X_C = frac150Ω$



          $0.00overline 6 + frac1X_C = 0.02$



          $0.02 - 0.00overline 6 = 0.01overline 3$



          $frac10.01overline 3 = 75Ω$



          $ X_C = frac12pi fC$ , pluging in 1MHz for $f$ ,



          $ 75Ω = frac12pi cdot 1Mcdot C$ and solving for C:



          $ 75Ωcdot 2picdot 1M = frac1C$



          $ 471,238,898.038 = frac1C$



          $C approx 2.2$nF



          You can also add a small speed-up cap across the unlabeled resistor on Q18-A's base. However, note in the datasheet that it specifies the maxiumum $t_on$ and $t_off$ of 4µs. $frac14µs$ = 250kHz, not 1MHz!







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 7 hours ago

























          answered 7 hours ago









          rdtscrdtsc

          5,4873 gold badges13 silver badges39 bronze badges




          5,4873 gold badges13 silver badges39 bronze badges














          • $begingroup$
            Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
            $endgroup$
            – calcium3000
            7 hours ago
















          • $begingroup$
            Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
            $endgroup$
            – calcium3000
            7 hours ago















          $begingroup$
          Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
          $endgroup$
          – calcium3000
          7 hours ago




          $begingroup$
          Thanks! I'll try the speed-up cap. Q18 is a dual, prebiased NPN so the base resistor is not available. And I saw that $4 mu s$ on the datasheet! Very confused how they can claim 1 MHz -- I thought maybe they had some trickery like, "Well, our output circuitry can switch at 1 MHz -- the LED just can't switch that fast. But imagine if it could!"
          $endgroup$
          – calcium3000
          7 hours ago


















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