Hello my friends,
I have to use an AD8599 and I tried to measure the CMRR with Vcc=+/-5V with -2V<Vcm<2V (as noted in the datasheet). The result is about 110bB (vs 120dB Min). I tried using conditions Vcc=+/-15V with -12.5V<Vcm<12.5V and it's the same. I carry out these measurements with DC conditions.
Could you help me if you know conditions to reach the value given in the datasheet?
I tried 2 bias methods:
In accordance with MIL-STD: Vcc+=+7V; Vcc-=-3V; Vin=+2V and Vcc+=+3V; Vcc-=-7V; Vin=-2V
with a direct method (instrumentation amplifier between In+ and In-): Vcc+=5V; Vcc-=-5V; Vin=-2V then Vin=+2V
The Delta of the Vio is about 10µV with -2V<Vcm<2V.
Then, I downloaded the Pscipe model on the AD website... I've got the same results!
I have another comment, I think there is a mistake on the datasheet Rev. C (page 3): the condition of the Open Loop Gain with Vsy=+/-5V specifies -11V<Vo<11V... Wahoo... Beyond the rail!!!
Thank you for your answers.
I agree that 110dB is really low compared to the 120dB minimum as stated in the datasheet. Your test conditions seem to be correct. Are you comfortable sharing your test circuit schematic? Maybe I can take a look at it so that I can help you further.
And for the Open Loop Gain condition on the datasheet, thank you for pointing that out. I have already forwarded this issue so that it could be corrected ASAP.
Could you please tell us what is the exact test method ADI use for the CMRR measurement of the AD8599 ?
I have contacted you through email.