OPA858DSGEVM: Max possible amplification

Hy Twahidi,

higher gains are possible, at the cost of lower bandwidth:

tayeb_opa858.TSC

Kai

Hi Kai, 

Thank you very much for quick response.

Essentially I was hoping to amplify an input signal of 60uV ac by around 100x, if not more @ approximately 40MHz. The ideal power supply would be to use dual supply +/-2.6V, though if single supply is preferred that can work as well.

I had a few questions with the schematic above.

Is V4 a possible DC replacement for VG1 as the input signal or does is serve a different purpose?

I have the below configuration setup to amplify an input signal of 60u. I have an OPA858 eval board to test out the validity of the layout but I was wondering if you could give me your opinion on the few alterations I made.

When I measure the output voltage via the AC Analysis <Calculate Nodal Voltage> I get 13 mV

When I measure the output voltage via the Transient Analysis  I get 1V output

Admittedly even 13 mV is a good output (higher is always better), but based on the gain diagram, it should be an amplification of roughly 100x

tayeb_opa858(1).TSC

I apologize for the possible overload of information, but any help with the layout would be greatly appreciated.

Best regards,

Twahidi

Hi Twahidi,

Tayeb Wahidi said:

Is V4 a possible DC replacement for VG1 as the input signal or does is serve a different purpose?

V4 is from the TI reference design which I adopted because I was in a hurry. Of course, you don't need V4 at all and you can easily power the OPAmp by a bipolar supply voltage.

Tayeb Wahidi said:

When I measure the output voltage via the AC Analysis <Calculate Nodal Voltage> I get 13 mV

This was the DC offset voltage at the output.

I have modified the schematic a bit:

tayeb_opa858_1.TSC

Keep in mind that the input offset voltage of OPA858 can be up to +/-5mV. So, with a gain of 100 you will see an output offset voltage of up to +/-500mV being superimposed to the wished output signal. You may want to add a high pass filter at the output of OPA858 to remove this DC offset voltage?

Where is your input signal coming from? What is the source impedance? Is it a sine wave as in my simulation?

Kai

Hi Kai

The signal source is a photodiode and the source impedance is approximately 50 ohms (SMA connector) and yes the sine wave is correct.

Thanks

Twahidi

Hi Kai

I was trying to set up a HPF for the 100x amplifier layout, but the best I got was approximately 30 degrees (though that may be due to my own limitations)

I decided to  aim for a lower amplification in the hopes of finding a balance between reasonable output and minimal phase shift.

Based off the ratio between input and feedback resistance, the amplification of the below schematic should be around 30x, yet surprisingly I get almost 100x amplification regardless ( 60uV - 4.54mV) with a phase shift of only 12 degrees.

tayeb_opa858_0427-4.TSC

I am going to tryout the layout on my eval board and see how similar the results are to the simulator.

Best regards,

Twahidi

Hi Twahidi,

photodiode signals are usually amplified with a TIA (Trans-Impedance Amplifier), as shown in section 10.1 of datasheet of OPA858.

Kai

Hi Kai,

Hyp TIA amplifier.pdf

The above schematic is the TIA that was used on our custom board-I am not sure how well it was tested prior to final PCB setup-so it is essentially what I was given to test and try to solve.

Based off the datasheet the photodiode signal is to the negative input of the opa858 followed by a voltage generator signal to the positive input of the OPA858. I was wondering if you could give me an idea why there would be 2 inputs and whether that was a requirement-or optional.\

Also if the TIA performs better when using single supply over dual supply.

Also in theory, how one would emulate the photodiode on TINA

tayeb_opa858_0427-4(1).TSC

The phase shift is about 180, but I am assuming that it would be related to the negative input.

Admittedly I should have specified that the goal was to use the OPa858 as a TIA from the beginning-I apologize for the confusion.

Thank you once again,

Tayeb

Hi Tayeb,

you might want to watch these TI's training videos on TIAs:

https://training.ti.com/ti-precision-labs-transimpedance-amps-introduction

https://training.ti.com/high-speed-transimpedence-amplifier-design-flow

Kai