There are many models of 7 segments panel meters on Aliexpress, cheaper than one can build. Various colors and segment sizes, most of them with 4 digits. Some are single indicator, some are grouped in 2-3 lines for V, A, W. They usually have a trimmer to calibrate them.
If you can trust the specifications; it is not trivial to achieve 4 digit performance.
Yea, a $10 7-digit Ali/eBay voltmeter is not the same with a $$$ bench Keithley 7-digit voltmeter, obviously...
That is why i said that those X-digit cheap meters are nice but trust only a couple of decimals points and ignore the rest.
It is not even that. I really mean that 4 digit performance is not trivial, especially if cost must be kept low.
Even assuming a perfect ADC, any signal conditioning, like an input buffer to provide a high impedance input, is not trivial. 1 part in 10,000 requires 80dB of common mode rejection to keep the error less than 1 count, but that is a difficult specification to meet for the JFET or especially CMOS input amplifier needed for a high impedance input. (1) The CMOS ICL7135 and its predecessors were designed very cleverly to null the common mode error of their horrid CMOS input buffer as part of their automatic zero cycle. Other contemporary ADCs which did not do this had to use a very expensive external precision JFET amplifier instead.
The amazing part is that there are relatively simple discrete ADC designs from that era that manage it, but they are so difficult to understand that even then an integrated part was preferred. Today if I was designing something from scratch, I would use one of the many integrated delta-sigma ADCs (2) (3) with a microcontroller to directly drive a multiplexed display, although for a high impedance input this would still require great care of the input circuitry. Duplicating the performance of the ICL7135 is still not easy.
(1) There are some amazing (non-chopper) parts now that handily meet this requirement, but the best option for a long time was a discrete design, a LM308 or LM11, or later the LT1008/LT1012 bipolar precision low input bias current operational amplifier. Analog Devices, PMI, and Burr-Brown had suitable precision JFET parts if you could afford them.
(2) The LTC1043 switched capacitor building block could also be used as part of a discrete ADC to achieve this level of performance.
(3) There are sampling ADCs with the needed precision now, however they would require external processing to remove 50/60 Hz line noise. Instrumentation delta-sigma converters can be configured to do this internally.
Home
Help
Search
Login
Register
