OPA637 or 627 for I/V on non O/S 1541 - diyAudio
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Old 31st March 2005, 06:47 AM   #1
tubenut is offline tubenut  South Africa
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Default OPA637 or 627 for I/V on non O/S 1541

Hi all

I believe the difference between these two op amps is their stability at unity gain vs high gain (greater then 5)...
Which of the two would be best for I/V stage?
I am burning in 627 with a burn disc on repeat at the moment.
Is that the ight choice?
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Old 31st March 2005, 07:35 AM   #2
ABO is offline ABO  Netherlands
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When used as I/V converters, the loopgain is unity. This means that the opamp must be unity-gain stable.

The opa637 will not be stable in this application.

Edit: it's not the loopgain that is unity, but the feedback factor b. In normal applications, the gain is 1/b, which is unity in this case.
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Old 31st March 2005, 07:46 AM   #3
tubenut is offline tubenut  South Africa
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Thank you. I made the right choice from memory then. I struggle a bit with the search feature, I normally get best results from this forum but wih google... I just did this and have established that the 627 is a good choice. I do however need to remove a compensation cap if it is in the player. 22-47 pF apparently. Is this connected across pins 5 & 8 on a 5534?
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Old 31st March 2005, 09:35 AM   #4
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Pin 8 on 627 is not connected internally so it shouldn't make much difference.
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Old 31st March 2005, 10:12 AM   #5
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Quote:
Originally posted by ABO
When used as I/V converters, the loopgain is unity. This means that the opamp must be unity-gain stable.

The opa637 will not be stable in this application.

Edit: it's not the loopgain that is unity, but the feedback factor b. In normal applications, the gain is 1/b, which is unity in this case.
It depends on the OP Z of dac. For example PCM63 = approx 1k
so if FB resistor = 2k then you only need to put 1k from dac OP
(-IP opa) to gnd and it will be stable. Usually 637 will be stable
+5 or -4 gain.

The disadvantage is you can not use any LPF on I-V

cheers,

Terry
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