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# DDC232: Timing precision for switching between integrators

Part Number: DDC232

Hello there,

My system produces double-pulses (like 1 us each, separated by as little as 0.5 us) at a few hundred Hz.

I am wondering if the DDC chips (which we use extensively) could be used to capture 2 sub-pulses separately.

To that end I can imagine starting the integration well ahead of the double pulse, so that the switching between the integrators falls right between the 2 sub-pulses.

As a result I would have the 2 sub-pulses split into the A and B integrators (probably using non-continuous mode?).

Thus my questions:

- what is the resistive load of the current input (will it even allow sufficiently fast rise and fall times?)

- is the switching between the integrators fast enough (at most a few hundred ns) so I can time it between the 2 sub-pulses.

Thank you!

• Hi Martin,

Sorry for the delay replying but we honestly don't have experience with this kind of operation of the device. What you say is actually how I would try to make it work. I think we can safely say that switching from side to side will be fast (ns). The input impedance of the DDC232 will depend on the range. I am going to try to paste the data here and see how it comes out:

 Range 12.5pC 50pC 100pC 150pC 200pC 250pC 300pC 350pC Input % DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) DC Bias Voltage (mV) 0 0.118 0.114 0.105 0.104 0.102 0.096 0.096 0.099 50 0.523 0.579 0.648 0.718 0.786 0.858 0.92 1 90 0.838 0.941 1.107 1.206 1.326 1.45 1.59 1.72 Q (pC) 12.5 50 100 150 200 250 300 350 Current (nA) 78.125 312.5 625 937.5 1250 1562.5 1875 2187.5 Zin (kOhms) 10.2 2.9 1.8 1.3 1.1 1.0 0.9 0.8

Hope this helps,

Edu

• In reply to Eduardo Bartolome:

Thank you, Eduardo!

So switching should not be the problem, but with an input impedance around 1 kOhm I will have difficulty collecting all the charge from the large photo diode in ≤500 ns, don't you think? I assume the Frequency Response shown in Fig. 7 of the DDC232 data sheet does not apply in non-continuous mode, correct?

Can you suggest any solution?

• In reply to Martin Rommel1:

Hi Martin,

Discussed with a designer here... As you imagined, not all the charge will be collected. Nevertheless, the loss can be computed/corrected to a first degree. Things that will make this correction not exact are time errors (if the pulse position vs CONV changes) or if the input impedance changes with signal. On the second, in principle it seems very linear but that is for a given range of current (the different measurement points were taken integrating over the full period). If now the same amount of charge comes over a shorter period of time, the input current may be much higher and the impedance actually not be linear or even have some frequency dependency. I guess trying it is the only way, but may work...

The frequency response comes from the periodic integration time. If you change that, then it certainly can be different...

Best regards,
Edu