Negative reference limits performance (by Gerard Larooy)

Component: ICL7106

Category: Special function IC's

This started off as a good beetle which formed a DVM. It could operate off +/- 5VDC accepting analogue signals referenced to ground and driving an LCD display.

With "progress" it lost its large 40 pin DIP package and looked like a blob of black candle wax covering a myriad of fine copper tracks. This smaller, cheaper version is usually found in cheap digital panel meters operating off a 9 volt supply. Unfortunately the negative input terminal isn't tied to sensible places like -9 volt or supply centre tap. In fact it's referenced to +9 volt via a band gap type reference. This causes real issues when the panel meter is intended to be integrated into equipment and operated from the equipment power supply. If this problem is "cured" with a differential amplifier or VCCS scheme using garden variety components with no trim pots then it's likely that the overall accuracy won't be any better than the analogue meters that it was intended to replace.
A small isolated DC to DC converter will nail this issue but overall power consumption is increased.

Another problem is the lousy sample rate of 3 readings per second. Aliasing will occur with most practical varying signals. The book input filter usually 1M, 10nF won't be adequate. A more complex filter with much lower cut frequency is called for here when the reading bounces. This will slow down the response of the meter.

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