OPA657: output bandwidth less than expected

guanting hou

Part Number: OPA657

I've both simulated and tested a TIA cuicuit using OPA657. this circuit is made for a 30pF PD

simulated result is here

2layer TIA circuit is made ,layout is close to the evaluation board. I make a interface circuit (C1=4.7nF,C2=30pF) and use the VNA to test the bandwidth.

VNA results is here(Cf=0.25pF)

if I change Cf to 0.1pF, the bandwidth is 6.38MHz.

i wonder whats the problem ?

over 1 year ago

0 kai klaas69 over 1 year ago

Guru 140200 points

Hi Guanting,

this thread may interest you:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1064582/opa818-testing-tia-with-vna

Kai

0 kai klaas69 over 1 year ago

Guru 140200 points

Hi Guanting,

looks reasonable. Where do you see a problem?

guan_opa657.TSC

guan_opa657_1.TSC

Kai

+1 Michael Steffes over 1 year ago in reply to kai klaas69

Guru 48395 points

Keep in mind that surface mount thick film R's are about 0.18pF parasitic by themselves

0 kai klaas69 over 1 year ago in reply to Michael Steffes

Guru 140200 points

This appnote may be of interest:

https://www.vishay.com/docs/60107/freqresp.pdf

Kai

0 Michael Steffes over 1 year ago in reply to kai klaas69

Guru 48395 points

Tha app note is pretty thorough, but for RF work 100Mhz to 60GHz.

I had measured a range of thick film sizes on an HP4195 network analyzer using the 1 port adaptor with careful calibration. Each size came out to about 0.18pF. There are very low (.05pF) R's from Precision Resistive PRoducts, but not in the 100kohm region.

0 guanting hou over 1 year ago in reply to Michael Steffes

Prodigy 20 points

Goodmorning kai and michael.

thanks a lot ! if add the 0.18pF parasitic ,it makes sense.

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OPA657: output bandwidth less than expected