Trainsimpedance amplifier

[bareil76]

Hi,

I am designing a transimpedance amplifier using the lmp7721. I am using a circuit similar to figure 6 in https://www.ti.com/lit/ds/symlink/lmp7721.pdf.

However, I want to know if I can put Vref at ground; i.e. make the guard at the same potential as the ground?

Thanks for your help.

 

[keith1200rs]

Yes, but you will zero bias the photodiode if you do that. That may not matter to you, but it increases capacitance and reduces speed. However, for some large area photodiodes it is the only way to avoid high leakage currents.

Keith.

 

[bareil76]

Yes I need zero leakage current.. so I need no bias on the photodiode. Thanks

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What do you think of the reed relay for a dual gain transimpedance amplifier. (2 resistor in parallel, and the second smaller resistor as a reed relay in series)

http://www.digikey.com/product-detail/en/9814-05-00/306-1159-5-ND/586522

 

[keith1200rs]

My concern about using relays would be the problems of stray pickup. However, while I haven't used those relays in a transimpedance amplifier, I have used them for gain switching high impedance circuits with good results.

Keith