Response of Op37

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saurabh17g

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We are using Op37 opamp to amplify the output of AD9833(frequency generator). We have built a non inverting configuration and the gain is 5(Rf=220k and Rin = 56k).
Our expectation is that this gain should be constant from 1Hz to 1MHz. But after 300Khz, we are finding getting sudden amplification and the output appears as clipped sine wave.
This behavior continues till 1MHz and we get a sine wave at 1MHz with distorted (almost triangular) shape.

GBW of Op37 = 63MHz. According to datasheet.
So theoretically,
it should give gain of 9 at 8MHz as well!
 

hi,
How are you connecting the Freq Gen to the OPA , via a capacitor, also how is the output coupled to scope.?
 
They made the OP37 fast by using a compensation capacitor too small so it cannot be used for low gain. Therefore its minimum allowed gain is 5.0 (look on its datasheet) but your gain is lower at only 3.9 so it peaks like you see or it oscillates.
 
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i m not using a capacitor at the output of AD9833 since capacitor shorts the higher frequencies and i want frequencies upto 1MHz.
i m using a PCB not breadboard.
 
@ericgibbs
we are connecting the output pin of ad9833 directly to the non inverting pin of op 37
we have connected the output of the opamp directly to the DSO
 
saurabh17g said:
@ericgibbs
we are connecting the output pin of ad9833 directly to the non inverting pin of op 37
we have connected the output of the opamp directly to the DSO
Click to expand...

hi,
Is the input impedance of the DSO 600R, if not, try matching to 600R using a 'T' network.
As I read the datasheet the specifications are quoted for a 600R load on the output.
 
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How about stray caps?

Can you shows us how your design looks like with a schematic?

I see that the gain settings resistors are 220k and 56k? This is rather much if you want to have a MHz performance. Possibly you will also need a buffer for better drive capability. Personally I would choose 10k instead of 220k as feedback.
 
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