[SOLVED] ad811 CFB opamp, unusual behaviour.

[zoulzubazz]

Hey guys,

I have been looking to build a high frequency non-inverting opamp circuit. The intention is to use the amplifier at 10MHz but since I am working on a breadboard tests are being done at 200kHz 1Vp-p input.

Following is the datasheet of the amp:

https://www.analog.com/media/en/technical-documentation/data-sheets/AD811.pdf

the figure shows the cicuit used to measure and null the input offset voltage. with the circuit i was only able to trim the output voltage to about 540mV, suggesting an offset volatage at input to be around 54mV (gain is set to approximately 10). The datasheet states the maximum value of offset to be around 3mV.



Also when the non-inverting input is fed the 1vp-p signal (input terminated with 50ohm) the output is approx 10Vp-p but if offset by 5V DC. Any ideas why this could be? Thanks very much.

 

[crutschow]

You results sound kind of odd. I suspect some kind of high frequency oscillation.
What type of breadboard are you using? A plug-in board likely will not work.

 

[Audioguru]

Another breadboard problem. Solderless breadboards should be buried.

 

[zoulzubazz]

thanks. yes, it is a plug in type. but shouldnt that be fine for mesuring offset voltage with no input?

 

[zoulzubazz]

three devices and much misery later, it turns out it was an error in the data sheet. My bad actually for blindly following the pin assignments in figure 25, -Vs is shown to be pin 5 instead of pin 4

 

[schmitt trigger]

Many moons ago (late 1980s), I also found a mistake on a datasheet. From Samsung semiconductor. I actually mailed them (no e-mail back then) and they acknowledged the mistake.

Nowadays, it is a lot easier. You can provide feedback on the Analog webpage. Who knows, they may even give you some freebies for the trouble :wink:

 

[Audioguru]

Errors on datasheets?? Too many companies hire kids straight out of school and have them do jobs they know nothing about, with no supervision.