Problem using AD8629 as an ADC driver

I wanted a low drift ADC driver in a precision thermometer circuit so I used an AD8629 (please refer to attached pdf). The op-amp +ve input was fine (i.e. a dc level proportional to the thermistor resistance) but the output was oscillating at several hundred mV, at about 800kHz. The same thing happened using an LTC2052, but problem was fixed when I used an LMP7702. What was I doing wrong?

ADCDriverProblem.PDF
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  • Hi Harry,

    I was using a 1nF cap originally, but the AD8629 still oscillated. The frequency is about 800kHz. Subsequently, after I got the circuit working with the LMP7702, I increased the cap to 10nF. I'm averaging 4096 successive ADC samples at a rate of 100k samples/s, and I needed the increased capacitance to keep the ADC input stable.

    I've found an article in EDN, October 16, 2008 that describes this kind of op amp stability problem - I guess I need to learn a bit more op amp theory.

    By the way, there's an error on the schematic - pin 5 of the op amp is connnected to a +2.5V reference, not +5V(A) as shown.

Reply
  • Hi Harry,

    I was using a 1nF cap originally, but the AD8629 still oscillated. The frequency is about 800kHz. Subsequently, after I got the circuit working with the LMP7702, I increased the cap to 10nF. I'm averaging 4096 successive ADC samples at a rate of 100k samples/s, and I needed the increased capacitance to keep the ADC input stable.

    I've found an article in EDN, October 16, 2008 that describes this kind of op amp stability problem - I guess I need to learn a bit more op amp theory.

    By the way, there's an error on the schematic - pin 5 of the op amp is connnected to a +2.5V reference, not +5V(A) as shown.

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