I'm developing a battery monitoring circuit. There is a problem. I launch a survey of all ports ADG507. For the test, I connect only two power supplies instead of the first and second batteries. The second power supply is 0 volts, the first 6.5 volts. No problem, I read from the ADC value of 6.508 volts on the first port. I increase the voltage of the second power supply and read from ADC the correct value on the second port. But on the first port the measured value begins to decrease and continues to fall to zero with the increase of the voltage on the second port up to 7.347 volts. The measured voltage value on the second port is correct. On the first port is 0, but I did not change the value on the first port. The voltage value at the first port remained at 6.5 volts from the beginning of the experiment. What kind of miracles? Why is not the voltage at the first port measured correctly when the voltage at the second port rises? What's my mistake?
2018-05-30 10:53:17,702 INFO tcpm. MainThread connected 6.508 0.000 0.000 0.001 0.001 0.001 0.001 0.001
2018-05-30 10:54:39,502 INFO tcpm. MainThread connected 6.391 2.057 0.000 0.001 0.001 0.001 0.001 0.001
2018-05-30 10:56:11,119 INFO tcpm. MainThread connected 4.066 4.067 0.000 0.001 0.001 0.001 0.001 0.001
2018-05-30 10:56:50,075 INFO tcpm. MainThread connected 1.229 6.513 0.000 0.001 0.001 0.001 0.001 0.001
2018-05-30 10:57:42,671 INFO tcpm. MainThread connected 0.333 7.374 0.000 0.001 0.001 0.001 0.001 0.001
The whole scheme with a controller and other kit is big. The problem is in the multiplexer circuit or (and) its interface with ADC. I append the test scheme and a sequence of measurements.
Nothing works. Assembled scheme from the AD7705 datasheet page 41, assembled my scheme, with resistors at the mux input, without resistors at the mux input, with connection 0 mux and ADC to 0 battery and without, in buffered and unbuffered mode of ADC. The voltage measurement is still incorrect. The ADC measured value at one input of the multiplexer depends on the voltage at the other input. And when I turn off the power of the multiplexer the inputs of the multiplexer without input resistors very heavily load the battery. What I doing wrong? MUX inputs scheme? My previous scheme assembled on the relay gives stable results of voltage measurement. In theory, it is possible to develop this scheme on multiplexers, or it is better not to waste time and leave the circuit on the relay?
(the text has been edited) I do not plan to power the multiplexer from the top of the battery stack, because the all battery stack is 48 volts but the power supply of the multiplexer is 16.5 volts max. And I dont need it.The entire battery is connected to the charger and I do not need to get power from it. I tried to isolate the power supply circuit using differential inputs and a DC/DC converter. I just need to measure the voltage on each cell of the battery. And now I made the test circuit (added to post) to just to understand - why voltage behave strangely. If I could to understand this then the entire circuit with 8 batteries can be assembled without problems.
I use a two-channel power supply for testing instead of batteries and DVM Fluke 27II for measurement. Set A0=A1=A2=0V ENA=+5V to use only the first port mux S1A S1B. Now set the supply voltage CH1=6 volts, CH2=0 volts. On the digital voltmeter we have 0.790 volts. I'm increasing the voltage onCH2. CH1=6V CH2=3V, and on digital voltmeter we have 0.774V. Continue increasing the voltage on CH2. CH1=6V CH2=6V, and on digital voltmeter we have 0.757V Why is this happening? Why the output voltage of the multiplexer decreases with increasing voltage CH2? How to fix?