I am looking at the DDC264 current to digital converter chip and want to make sure I am understanding the input limits correctly. Operating in Range 3, the input range has a typical value of 150pC. If I am running at 500uS integration time, then is the current limit 150e-12 / 500e-6 = 300e-6 Amps? What would happen if I feed the input with a larger current, say 1mA? Do you have any current input chips that accept up to 1mA?
The DDC input is basically an integrator (an amplifier with a feedback capacitor). As current flows in this integrator, the output voltage increases, to a point where it would exceed the full scale of the ADC behind. That is your limit, a given charge on the feedback capacitor of the integrator. In Range 3, as you said, the limit is 150pC. With an integration time of 500us, this would be equivalent to a constant current of 150e-12/500e-6=0.3uA.
1mA of constant current will be way too much for this device, and no, we do not have a device able to handle that. Even our DDC112 that allows for the use of external capacitors would not be able to handle that (the capacitor would be too big and would make the solution unestable). Probably you would have to implement this with a discrete solution. Nevertheless, again, one should consider charge, not current. So, if your 1mA of current is pulsed, for a very short period of time, then there are things that can be done. Feel free to contact us if that is the case...
Thank you for the detailed response. The current would normally be pulsed, but there may be rare occasions when the current will jump to 1mA for a few seconds. I am working with APDs and the range of current measured has been from about 10nA noise floor to about 100uA with the optical power I am using. But, the current can go to 1mA and beyond if the laser power or APD bias is high enough.
I have thought about putting in some kind of current limit or automatic shutoff switch between the APDs and ADC if the current goes too high, but with 64 devices, there may not be enough room to limit each one independently.
Thank you for the support. If you have anything to suggest, I 'll be happy to hear it. If not, you have already been a great help, thank you,
Those current levels probably would damage the inputs of the DDC. Do you have a diagram of how do you connect the APD? I mean, is it AC coupled or DC coupled? I am asking because I guess that if it was AC coupling, the bias resistor would act as quenching? I am a bit rusted on all this... And I guess that your HV supply for the APDs does not have clamp either and even if so, maybe you decap would provide the current for short period of time...
Anyhow, I am sure you thought of all that... Not an expert on protecting any kind of inputs... but a thought would be to use an external diode in every input to ground (anode on the input, cathode on the ground). Usually that input is very close to ground, so, shouldn't affect performance except for the increase in parasitic capacitance. As the current increases, it will take the input of the DDC out of bias, increasing the input voltage and eventually exceeding the turn-on voltage of the diode, clamping the input (and current) to ground. I have not tried it, so, you would have to do the experiments...
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