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LT1328 powering & photodiode multiplexing

Category: Hardware
Product Number: LT1328

Hello,

I've 2 questions regrding LT1328 frontend.

1. Is it important to uso low-noise LDO for LT1328 powering ?

2.Will LT1328 performance be degraded if connecting 8 different photodiodes with 8 to 1 analog multiplexing... or a better way is to use one LT1328 per photodiode and then multiplex the digital outputs?

Thanks.

Sincerely,

Pavel.

Parents
  • Hi Pavel:  I don't see a supply rejection vs frequency plot in the data sheet.  I never had any supply issues with it over my 24 years of looking at the part.  But the input stage is quite noisy, and it sometimes spews output junk because the noise is overcoming the intentional offset.  The way to increase the offset is to put a weak pullup on the BIAS pin to 5V; 3.01Meg is a good start at overcoming its own noise.  In noisy environments (like automated test equipment) some customers have experienced grief.  But in those cases, success was always achieved simply by strengthening the pullup current.

    The problem with adding extra stuff on the front end is that it adds capacitance.  The demo board comes with a fairly large photodiode, 70pF as I recall.  And it's a wide angle photodiode.  If you used multiple SFH213 with high optical gain and low capacitance, you might get good performance.  But in your case, it would be the MUX capacitance in parallel with one photodiode at a time.

    By the way, I have successfully used the DN399 technique to reduce input noise on LT1328.   Are you planning to do 4ppm?  Using full rate 4ppm, the LT1328 becomes sensitive to different power levels at different codes.

Reply
  • Hi Pavel:  I don't see a supply rejection vs frequency plot in the data sheet.  I never had any supply issues with it over my 24 years of looking at the part.  But the input stage is quite noisy, and it sometimes spews output junk because the noise is overcoming the intentional offset.  The way to increase the offset is to put a weak pullup on the BIAS pin to 5V; 3.01Meg is a good start at overcoming its own noise.  In noisy environments (like automated test equipment) some customers have experienced grief.  But in those cases, success was always achieved simply by strengthening the pullup current.

    The problem with adding extra stuff on the front end is that it adds capacitance.  The demo board comes with a fairly large photodiode, 70pF as I recall.  And it's a wide angle photodiode.  If you used multiple SFH213 with high optical gain and low capacitance, you might get good performance.  But in your case, it would be the MUX capacitance in parallel with one photodiode at a time.

    By the way, I have successfully used the DN399 technique to reduce input noise on LT1328.   Are you planning to do 4ppm?  Using full rate 4ppm, the LT1328 becomes sensitive to different power levels at different codes.

Children
  • Thanks Glen,

    The application isn't infrared, it's in visible band. The coding isn't 4ppm, but S/PDIF (for transmitting audio), the datarate is about 1Mb/s. The photodiode we plan to use isn't selected yet, there are about 15 in the shortlist with 0V terminal cap varying in range 50pF ... 1500pF. According to the simulation in LTSpice, 1500pF is still Ok. That's why there was question on analog mux (in order to avoid using multiple LT1328 to test different photodiodes). Concerning powering, at your glance is it Ok to use one 500mA normal LDO (without particular noise ratings) for powering multiple LT1328 (perhaps filtering power interconnections with ferrite beds) ?