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Measure VCE/IC and BVE/IB using DAC

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Fab64:
Hi all,

I need some help from you expert engineers.

I need automatically measure VCE / IC and VBE / IB of a big bunch of transistors under specified Vcin and Vbin. See attached circuit diagram #1.

Since Vcin and Vbin can be either positive or negatice voltages I chose an ADC (ADS122C04, http://www.ti.com/lit/ds/symlink/ads122c04.pdf) capable to work with such voltages.

To stay in the ADC's maximum voltage ranges I tested two different solutions:


* Use two voltage dividers one for AIN0 and the other for AIN1. But, any small mismatch between resistors values leads to very high errors.
* Use a virtual ground tied to AIN1 and a single voltage divider for AIN3. This works well to measure VCE and IC, I get values very close to what I measure with my bench multimeter (see output #1).
Problem is with IB and VBE. Following the same approach used for IC/VCE I should tie VGND to the base, shorting base and collector.

To overcome this issue, I used an IC switch (ADG1636, https://www.analog.com/media/en/technical-documentation/data-sheets/ADG1636.pdf) to switch AIN0, AIN1 between collector and base. I chose that because the low series resistance.

See attached circuit diagram #2.

Everything works well up VC = 2.6 - 2.7V after that the measured current get smaller than the actual value  (see output #2).

I understand that the switch has a series resistance which get larger when voltage increases but, since the current on AIN0 and AIN1 is quite small, I thought it shouldn't impact the measurements.

I stuck here.

I need any advice either to get rid off the switch and read VB/IB without shorting base and collector or make the switch working up to  larger voltages.

Thank you in advance.

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