OPA657: OPA657 minimum trans-impedance gain

Well of course you can, just need to recompensate with feedback C, datasheet tells you how to set Cf. 

Hello Vinod,

To clarify, this plot is not a representation of the achievable trans-impedance gain of the device. All this is showing is the frequency response of the circuit trans-impedance circuit in the first page of the datasheet (shown below), and the gain is reduced due to the bandwidth limitations of the circuit. 

Changing your feedback resistor (and therefore your trans-impedance gain) will give you a different frequency response plot and can certainly be done. Like Micheal mentions, you can use section 9.2 of the datasheet if you need help designing for a different trans-impedance gain.

Best,

Hasan Babiker

Hi,

We tried in actual boards but we are getting oscillation (See attached waveform). This problem is same as discussed in below thread but no solution is suggested there also.

https://e2e.ti.com/support/amplifiers/f/14/t/646311?tisearch=e2e-sitesearch&keymatch=opa657%252520oscillation

Please tell how to resolve the issue. We have tried with multiple capacitor values but oscillation never goes below 100mv (p-p).

regards

Vinod Sharma

Vinod, these are helpful post guidelines, you have an oscillation, no way to resolve without details, 

Suggestions for effective circuit help requests –

1. Where your question has a specific circuit/device involve, be sure to show the full circuit and more preferably a TINA schematic link. Debug usually involves circling the device pins looking both directions for interactions. What is driving your input – what is your load?
2. “It doesn’t work” leaves a lot to be desired. Be as specific as possible where your current implementation is falling short of expectations.
3. Eventually, any design has some target performance metrics. Detailing those will go a long way towards reaching an effective end solution. This might require changing device solutions.

Hi Michael,

Sorry for not giving details. I though it is a general problem as there are two three threads for the same problem but none of them has resolution.

With an grounded input also we have observed oscillations. Is this due to high bandwidth with Low gain Rf = 200? There is no problem at higher gain i.e more than 1k.

Please see below schematic.

regards

Vinod Sharma

that circuit makes little sense if it is really transimpedance - where is your detector diode? (capacitance) also, do not probe right on output, probe capacitance will normally cause problems

Hi Michael,

To debug the issue we have removed detector diode & grounded the input at 120 ohm.

Also we have a 50 ohm at output. It is not shown in schematic.We are not directly probing at opamp output. 

Without any detector also we are observing oscillations.

regards

Vinod Sharma

Hi Michael,

Thanks for the sharing pdf files. I will go through them.

Meanwhile please tell why we are not facing this issue at higher trans impedance gain > 1k. Also at Rf=200 why we are not able to get smooth output with cf setting as per datasheet section 9.2. 

It is very surprising that with a grounded input also it is oscillating. But at higher gain it works perfectly.

regards

Vinod Sharma

Hello Vinad,

Could you please clarify which circuit you are currently using? The one in the previous thread you posted and the one in your TINA simulation are very different. Like Micheal mentioned, it would also help if you added your input current and capacitance to your TINA file. Also neither circuit seems to be put in the typical trans-impedance configuration (like the one shown in my previous response), could you please clarify what you are trying to achieve?

Best,

Hasan Babiker

Hi Hasan,

1. I am using the circuit the one shown in picture in this thread. Our diode, input capacitance is before 120 ohm resistance. In this configuration we observer problem first time.

2. Then to debug the issue , we removed input diode and grounded 120 resistance. In this case also we are getting oscillations.

I want to point out that that at low Rf value this opamp start oscillation with diode, without diode, open input, grounded input in all cases. Our aim to use this opamp for tranimpedance  only with a Rf=200.

regards

Vinod Sharma

Hello Vinod,

What is the capacitance of your diode? and why are you adding a 120 ohm resistor to your trans-impedance circuit?

Best,

Hasan Babiker