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Re: AD797 op-amp question : distortion cancellation

Posted by RJM (A) on May 27, 2002 at 10:04:16

In Reply to: Re: AD797 op-amp question : distortion cancellation posted by eagle1 on May 27, 2002 at 06:37:52:

"Check page 12 fig. 41 b. Table IV."

The two capacitors C1 and C2 in this table are related to CC and CN. C2 is CN and goes from pin 8 to pin 6. C1 goes from pin 8 to ground. It is an external decompensation capacitor that effectively reduces the value of the internal compensation capacitor, CC, 50 pF, extending bandwidth when high gains are used.

C2/CN is completely external. No internal 50 pF is included, hence, 50 pF minus C1 must be added.

Datasheet quote: "The unique design of the AD797 provides for cancellation of the output stage’s distortion (patent pending). To achieve this a capacitance equal to the effective compensation capacitance, usually 50 pF, is connected between Pin 8 and the output (C2 in Figure 41). Use of this feature will improve distortion performance when the closed loop gain is more than 10 or when frequencies of interest are greater than 30 kHz."

The interesting thing about the AD797 is that no-one I have ever come across recommends it, and yet it seems ideal for so many applications, especially a MC phono preamp. The advice is always "use the AD811 or the LM6172", both comparable to the AD797, though with much higher slew rates. The AD797 has the lower offset voltage, though.

So, is the AD797 a dog that only looks good on paper, or is it a hard-to-control thoroughbred? What, I wonder, is the problem?

I want to believe ... like so many before me I bet ... that the AD797 has potential. Right now I'm thinking: run the whole circuit at low impedance, taking full advantage of the chips low distortion drive. And, make sure slew rate limiting never rears its ugly head: make sure the gain of the output stage rolls off cleanly at high frequencies. And, make sure the output stage is configured to drive capacitive loads.

RJM

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