# simple voltmeter project

Discussion in 'Electronic Basics' started by roy davidson, Jul 4, 2005.

1. ### roy davidsonGuest

the circuit is more to demonstrate the principles than be accurate. I'm
trying to keep it as simple as possible, and the simulator I am using,
crocodile technology has a limited number of ics. I'd like to run the system
off a 9v supply, when the circuit is built we'll use a battery. I have a
circuit using a clock, binary counter, decoder, 8 seg display and DAC. I'm
using a summing amp for the dac. This is where my problem comes, the DAC
needs a dual supply.

Is there a way round the dual rail needed for the DAC ?

Do I have to split the 9v supply to get a dual rail? Use diodes?

here's my two rail starting point
http://www.roydavidson.ic24.net/voltm.jpg

cheers roy

3. ### John FieldsGuest

---
It looks like the problem is having to take the inversion in order to
do the current summation and get the true current to voltage
conversion in the DAC's input opamp, and then having to do the second
inversion to get you right-side-up again so you can do the successive
approximation.

Since you say accuracy isn't all that important, I'd cheat and sum the
DAC's input currents in a smallish resistor and then use a single
opamp as a non-inverting amp with as much gain as you need to make the
SAR work right. Like this:

DAC3>---[R4]----+--------|+\
| | >----+--> Out to SAR comparator
DAC2>---[R3]----+ +--|-/ |
| | |
DAC1>---[R2]----+ +---[R6]---+
| |
DAC0>---[R1]----+ [R7]
| |
[R5] |
| |
GND>------------+-----+

The smaller R5 is, the smaller the error will be in your
current-to-voltage conversion, but the greater the gain you'll need to
get the output to the SAR right. Unless you scale the reference
voltage input to the SAR's comparator. In any case, the gain of the
opamp, above, will be:

R6 + R7
Av = ---------
R7