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Using the AD844 as an I/V
Using the AD844 as an I/V
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Old 22nd June 2018, 10:11 PM   #2231
Hierfi is offline Hierfi  Canada
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Join Date: Oct 2012
Location: London, Ontario
Hi Joe,
My reason for asking is to determine if the AD844's input stage has enough class A current capability that it can be used as an output buffer in lieu of a BUF03 or the AD844's normal output buffer. In other words using only the input stage of a second AD844 with input to pin 3 and the output taken from pin 2, with its Tz pin 5 connected to ground.

From the thread the output buffer in the AD844 isn't necessarily a good one. Having done experiments with and without the AD844's output buffer it appears susceptible to oscillation of the kind that can become rectified and reflected back into the audio range. Given that oscillations normally occur under feedback conditions it would seem that the current limiters in the output buffer can be responsible. In other words, if high frequency transients exist at the Tz node, as either coming from the input, from the power supplies, or from ground through the RC network that members are connecting to this node, this can affect or activate the current limiters. This necessitates a requirement for good power supply decoupling and general high frequency network considerations in any design. Ultimately the output buffer in the AD844 is a feedback amplifier.

The AD844 has already proven itself as an excellent device by members for use as an I/V converter, one that uses a unity gain input buffer. Given that the input buffer doesn't employ current limiting (as indicated by the data sheet warning not to exceed 5mA on the inverting terminal) it isn't considered prone to oscillations.

Another factor is that the input buffer is designed to be clean (being laser trimmed as well), as the voltage across the input terminals represent the reference points that generates feedback, hence are outside the feedback loop (the AD844 can't guess what needs fixing). This is not the case for the class AB output buffer, normally current limited to protect the device from damage, with feedback limiting its non-linearity.

Cheers, Gerrit
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