OPA189: OPA189

You are asking for a gain of 2000V/V, Fine - use a two stage OPA838, 1st stage at gain of 150V/V, 2nd at 15V/V

Hi Steffes,

Thanks for the response.

We hope 1 stage amplification is with gain of 150V/V is sufficient for oscilloscope to do measurement.

As per you your response, we understand that OPA189 is sufficient to do the amplification.

You say 0.1mV to 200mV out, that is a gain of 2000

Hi Michael Steffes,

our worry for choosing OPA838 is only input offset voltage  as well output offset voltage.

OPA189  posses low offset voltage.

Do we need to consider anything related to the offset voltage of the op amp.

Thanks 

Nithin

you also need to think about gain bandwidth product. 

at a gain of 150, the OPA838 is about 2MHz BW, the OPA189 will be far lower. 

The OPA838 is pretty good offset voltage wise, but you said 100Hz min, AC couple the gain then to reduce the DC gain for the offset voltage

Hi Nithin,

In addition to Michael's suggestion, you may be able to use OPA838 as first stage, and use OPA189 on the second stage, and AC coupled between two stages. Your op amp's BW may be limited to approx. 1MHz (may be acceptable per your requirements). 

Best,

Raymond

Hi Michael.

Can you please share the gain setting resistor configuration for gain of 150VV.

When we are simulating the Op-Amp with  6v/v gain 

we are getting waveform like below in which we observed that,

-ve peak is higher than +ve peak

Sharing the schematic also for reference.

Requesting you to please correct if any fault in the schematic.

You have not answered if you can be AC coupled and if so at what frequency

Hi Michael,

Sorry for not updating AC coupled frequency,

Frequency what we use is about 170KHz.

Requesting for your valuable points on above simulation results of OPA838

Thanks 

Nithin

So I am guessing that is a yes, so would a 100kHz high pass with that higher gain flat at 170kHz be ok? 

Hi Michael,

Our input signal frequency will be between 10Hz to 170KHz

 Therefore we are not sure in AC coupled amplifier 

Thanks 

Nithin

Ok Nithin, I went to your 1st message and it said 1Hz to 1MHz. The OPA838 can do 1MHz say at a gain of 201, here is that starting point design before we zero out the DC offset, I am running +/-2.5 supply - 

Then we add a servo amp using a really good low noise chopper - I set the high pass pole here at 0.2Hz. I know the V+ feedback looks odd, but the feedback through the OPA838 is inverting so this is correct. I also added a series 100ohm output to isolate the capacitance of a high impedance scope - don't need a 50ohm input in this case. 

And the AC response for this is pretty good, 

And then looking at the input spot noise, You can do better than this with higher power parts (LMH6629) but this is pretty good. 

And then the file, 

Gian of 201 with DC servo OPA838.TSC

Hi Nithin,

We haven't heard from you in a while. We hope the issue was resolved. I am going to close thing thread now, please reply to reopen if you have any additional questions / information.