From: Spehro Pefhany
Subject: Re: digital voltmeter and ammeter
References: <3E2AB7F8.email@example.com> <3E2ABEF9.firstname.lastname@example.org> <3E2AC2FE.email@example.com>
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Date: Mon, 20 Jan 2003 15:51:00 GMT
NNTP-Posting-Date: Mon, 20 Jan 2003 10:51:00 EST
On 20 Jan 2003 07:34:13 -0800, the renowned firstname.lastname@example.org
(Tim Shoppa) wrote:
>Pot-AY-toes, Pot-AH-toes. I like non-multiplexed displays, personally;
>less switching noise (though that will be a complete non-issue in a
I like 12 wires for 4 full digits with decimal points rather than
32. ;-) No problems so far with switching noise (10uV resolution),
in fact *too* quiet in one case.
>That said, are there any more-modern 3-1/2 digit all-on-one-chip panel
>meter type A/D->LED display chips with on-chip reference? Or is
>the modern solution to use a PIC?
Maxim has some, but the pricing puts them out of the market except
for naive domestic low-quantity production. No modern all-in-one
ones that are muxed that I know of (there's an old RCA 2-chip set),
and the 7135 4-digit one, but it's not all-in-one. There are more
these days for the LCD multimeters that are made in the millions, and
there are a couple of front end CMOS chips for those, which could be
used with a uP and an LED display.
PICs don't yet have delta-sigma converters on-chip, though you
can do a slope converter without too much trouble. Other micros
(AD, BB) do have these, but very expensive.
>The 7107 data sheet tells you how to cut this for power dissipation
>reasons (although "too bright" is a complaint that I rarely hear.)
Their "solution" evens out the power dissipation, so their crummy
internal reference doesn't drift as much. You could PWM the display if
you like switching 200mA. ;-) Maxim does have one that has lower
segment current to address just this problem (but, again, the price is
too high for volume use..)
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