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Design >> Analog Design >> ADS830 and OPAMP
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Message started by teatime on Feb 28^th, 2011, 11:40am

Title: ADS830 and OPAMP
Post by teatime on Feb 28^th, 2011, 11:40am

Hello,

I made a simple PCB with a ADS830 60MHz parallel ADC from TI, fed by an OPA2691 (2681 equivalent) OPAMP using the circuit shown on page 9 of the ADS830 datasheet. I am feeding the ADC a 50MHz signal and reading the output with a logic analyzer.

I'm a digital guy fumbling in an analog world, and I wondered if anyone had any feedback on this circuit. (opamp joke...) I believe the goal of the circuit is to take an input with a 2volt swing around ground and move it to a 2volt swing around 2.5volts (ADS830 common mode voltage).

When the input is grounded the test board shows 2.1volts input to the ADC, close to the 2.5volt center voltage of the ADS830, but seemingly far off with a 2Vp-p.

Additionally, I added a 1/10 voltage divider to the opamp input to divide the input down (90K/10K resistor divider feeding the 499ohm R), but it really seems to mess stuff up. If I remove it I get much better results (the 2.1v at ground listed above).

Any advise would be greatly appreciated.

Title: Re: ADS830 and OPAMP
Post by loose-electron on Feb 28^th, 2011, 2:42pm

Generally the approach for this type of a circuit is to create a differential signal and use both inputs to the ADC, instead of tying one input to the common mode voltage.

Whats the BW of this system need to be?

Title: Re: ADS830 and OPAMP
Post by teatime on Feb 28^th, 2011, 11:04pm

Thanks for the reply. I think there is a differential mode example in the datasheet too. I'm sorry, I don't know about the bandwidth but I will start researching it now.