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ADL5304 Response Time

Question asked by ricky on Jan 18, 2016
Latest reply on Jan 31, 2016 by ricky

Hi All,

 

One of my customer is evaluating his prototype board which has ADG708 and ADL5304 Log Converter.

His schematic diagram is attached.

He has one issue with the ADL5304 for determining scan rate of photo diode sensors by ADG708.

He made test operation as next;

 

Test Setup: Only PD1 is connected to S1 terminal of ADG708, and other terminals, S2 through S8, are left open.

 

Operation: First ADG708 switch is connected to S2, it means INUM terminal of ADL5304 is open. Then ADG708 is select S1 for 1msec interval, and select S2 again. The output of ADL5304, VLOG terminal shows square waveform as shown in below schematic. Delay is around 3msec.

 

Question:
1) Is this 3msec delay reasonable one?
2) In the final circuit, all photodiodes are connected to S1 through S8, and the ADG708 will scan them for measurement. In this case, should the scan rate be slower than 330Hz?
3) Is there any way to shorten the delay time (Response Time)?
4) When the R203 (1Mohm) is connected to INUM as shown in the schematic below, the delay is shortened to around 100usec, although offset voltage appeared on output. Is this modification acceptable, if output offset is accepted in application?

 

Thank you for your help,

Ricky

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