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I am having trouble with the 7107 IC. I have 5 of them on my PCB: 3 being used as current measurement and the other 2 are being used as a voltage measurement.

Only one of the 5 are displaying readings on the 7-segment display (may not be calibrated correctly but is showing something). The other 4 are the 7-segments are all blank. I have attached the circuit diagram below.

I have check the all 7-segments are working with the test feature on pin 37. The clock is oscillating at 40-43KHz (check with oscilloscope). The difference I am seeing with respects to the one working versus the others is that 100nF connected to cref+ and cref- on the working unit is pulsing about 1.3V, the others have no pulse or very low (mV) pulses.

I really starting to believe I have a bad batch of IC's. Please provide any tips or recommendations. Thanks in advance.

enter image description here

Michel Keijzers
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JOHNDOE
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1 Answers1

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You are almost surely seeing the display 'blank' because you're ignoring the most significant digit (AB4 is open). The meters are in overrange (displaying + or - 1bbb where b is blank).

The 7107 works much better with a negative supply (+/-5V is ideal). If you want to use it with a single +5V supply you have to pay a lot of attention to the common mode range of the inputs, so it has to be more than 1V or something like that (read the datasheet) relative to analog COMMON.

Spehro Pefhany
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  • I trying to read 24V input however the will be volt drop across the 1M. – JOHNDOE Nov 05 '18 at 15:58
  • You have Vin(-) grounded. That is outside of the common mode range of the inputs unless you give it a negative supply. – Spehro Pefhany Nov 05 '18 at 16:11
  • can i get to work with single +5V only? How? – JOHNDOE Nov 05 '18 at 16:31
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    @JOHNDOE See the data sheet - it shows a charge-pump circuit using a CMOS inverter chip, a couple diodes, capacitors to generate a -ve voltage. – glen_geek Nov 05 '18 at 16:40
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    As @glen_geek said. Or use a TPS60400DBVT, ICL7660 etc. charge pump to generate -5V regulated (my usual choice). – Spehro Pefhany Nov 05 '18 at 16:53
  • The other option is to try to offset the voltage to get it within the common mode range, but that's more effort and usually won't yield as good results in terms of noise. – Spehro Pefhany Nov 05 '18 at 16:54
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    So a negative 5 on pin 26 will make it work. Wish I knew this before I made the board. Hehehe guess I am always learning – JOHNDOE Nov 05 '18 at 17:10
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    Some learning is more painful than others. And the painful varieties are often avoided if you read the entire data sheet. – WhatRoughBeast Nov 05 '18 at 18:46
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    Note that you probably only need a single charge pump for the set of DVM chips if you use one like the 7660. The draw from the minus supply is only about 1mA each. – Spehro Pefhany Nov 05 '18 at 18:52
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    @JOHNDOE - If this answer solved your problem, remember to upvote / accept :-) – SamGibson Jan 02 '19 at 17:38