[time-nuts] 5 Digit Panel meters

[Robert LaJeunesse]

The similar meters I have are based on the Microchip MCP3421, a tiny 6-pin SOT 18 bit ADC with internal 2.048V 15 ppm (typ) reference, differential inputs, and a PGA. Input span is +/-2.048V at minimum gain to +/-0.256V with a PGA gain of 8. So at max gain 1 LSB is about 2uV, although I'd be surprised if it was that quiet (the spec at PGA=1 is 1.5uV typical, no max given). Initial accuracy is not great at 0.35% max, but that can be trimmed out externally or via software. For an 18 bit ADC with a decent reference it's actually a rather good deal at under US $2 (qty 25+). Just don't expect it to be an accurate 18 bits over a wide temp range, references like that don't sell so cheap.

Bob L.

ref: https://ww1.microchip.com/downloads/en/DeviceDoc/22003e.pdf

> Sent: Monday, September 07, 2020 at 1:15 PM
> From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Discussion of precise time and frequency measurement" <time-nuts at lists.febo.com>
> Subject: Re: [time-nuts] 5 Digit Panel meters
>
> Hi
> 
> So, *assuming* there is no other ADC on the board ….
> 
> DC performance of the STM8 ADC’s is somewhere in the 8 to 9 bit range as far
> as ENOB with significant averaging. For the sake of simplicity, let’s say they *do*
> make it to 10 bits. 
> 
> But … do we get the 10 bits?
> 
> The range on the meter is 33V. The reference to the DAC is (likely) 5V. You just lost
> nearly 3 bits from that scale mismatch.  At the 5V level, you would be running just over
> 7 of your (ideal) 10 bits. 
> 
> Net result is that you are trying to check a 16 bit gizmo with a not quite 8 bit device.
> 
> Bob