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ADG728 Analog MUX interference between channels

Question asked by TANKUTACAR on Oct 2, 2013
Latest reply on Oct 7, 2013 by TANKUTACAR



i am struggling a problem on ADG728. I am trying to measure optical density over a photodiode array of 8 photodiodes(You can view the image i attached). My problem started with my realization of false readings over channels. When i connect the Analog MUX's S1 pin to D output(via I2C command) and open circuited all other S(2-8) pins on both ADG728 itself(via I2C command) and also on the hardware(picking up the header connections existing between photodiodes and AMUX's S(2-8) pins) i make a correct reading for channel 1. But, if i connect physically a second channel pin(that is already in OFF state) to the photodiode array, (it does not make any change for which channel to use), the reading i am getting from channel 1(the only ON channel) changes dramatically.


After experimenting this behaviour from various channels, i made some measurements with my Fluke 289 multimeter. I disconnected all photodiode header(physical) connections and tried to measure the resistance values between channels on some conditions.

For example, after powering up(+5VDC) while all channels are in the OFF state, there is approximately 60-70 Mohm between each channel. This value is satisfying i guess(compared to the infine value(open circuit) that it should be in ideal MUX). The problem is that, when you turned ON only one channel(via i2c command), for example connect S1 pin to D, the resistance between that turned ON channel and any other OFF channel decreases to approximately 200-300 Kohm. In fact, that value can be measured with multimeter in one direction. That means when i connect the red(positive) probe of the multimeter to the ON channel(S1) and the black(negative) probe to the other one of OFF pins(S2-8). If i change the measurement nodes(black probe -> S1 , red probe -> other OFF Sx pin), i measure infinite (open circuit) value that it supposed to be.


I think this 200K value between ON and OFF channel causes the incorrect measurement i mentioned above. And i guess, this behaviour is unchangeable since it is the nature of ADG728. Can you advice us any other parts which does not have this drawback?? We still prefer i2c communication and small ON resistance(approximately 2.5ohm or as well).


Thanks in advance. Looking forward to hearing your comments and advices...


Tankut ACAR

Design Engineer



Message was edited by: TANKUT ACAR