TPS61391: TPS61391: current monitor connected to transimpedance converter

Christian_Fischer

Part Number: TPS61391

Hi together,

at the moment we are busy designing an APD power supply with output voltage trimming depending on sereveral environmental conditions. Voltage adjustment works as expected using and external DAC. But we struggle using the current monitor outputs.

Instead of using resistors connected to GND to convert current into a voltage we want to use a transimpedance converter as shown in the picture below:

OPamp ist single supply powered with 3.3V. Reference voltage on the p-Input is 1.0V.

Are there any restrictions when conneting to a amplifier like that? Do we have to keep anything in mind that is not part of the TPSxx's datasheet like limits for current sourcing or sinking or anythng else?

We are looking forwad to your answers?

Thanks for your help and yout hints in advance.

Best

Christian

over 4 years ago

0 Yichen Xu over 4 years ago

TI__Expert 4760 points

Hi Christian,

Why do you want to use the amp? If so, you can not monitor APD current directly. You can refer the application note below for choosing resistor.

https://www.ti.com/lit/an/slvaeh5/slvaeh5.pdf?ts=1630906702304&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS61391

0 Yichen Xu over 3 years ago in reply to Yichen Xu

TI__Expert 4760 points

Hi Christian,

Is there any update?

0 Christian_Fischer over 3 years ago in reply to Yichen Xu

Prodigy 100 points

Hi,

sorry for te delay but we had some problems in our IT so there was no chance for me to answer.

In the meantime we adjusted gain of our circuit so that we are able to get correct output.

We already know that appnote but for some reason we think it is more practical to use transimpedance converter. On our point of view there should be no difference between using a resistor connected to GND and an transimpedance converter with working point doing the same. Unless we run in to trouble becaus TPS monitor output has some restrictions we do not know.

Can you please give us more details concerning that?

Thanks Christian

0 Yichen Xu over 3 years ago in reply to Christian_Fischer

TI__Expert 4760 points

Hi Christian,

I don't find advantage using the amp. Do you want to clamp the MON pin voltage to 1V? It is unnecessary since you can choose the appropriate resistor to avoid over voltage on MON pin. And if you using the amp, it is easy to output 0V if the APD current is high.

0 Christian_Fischer over 3 years ago in reply to Yichen Xu

Prodigy 100 points

Hi,

thanks four your answer. That means you do not see any restriction when uisng the opamp instead of resitor?

Of course it could also be possible to use a resistor to convert monitor current but in our application we have to be possibly measure very low APD currents (only a few nA). For this we have to calibrate the system so that zero- Bias aof around 20uA is known.

Thats means we need an additional amplifier to gain the signal. Idea is to combine requirements and directly use a transimpedance converter to adapt to ADC input with adjustable gain.

Best Christian

0 Yichen Xu over 3 years ago in reply to Christian_Fischer

TI__Expert 4760 points

Hi Christian,

For nA current it may be hard to measure. You need even consider bias current and offset current of the transimpedance converter. And make sure the Rf is proper to avoid near to 0V output.

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TPS61391: TPS61391: current monitor connected to transimpedance converter