Sean Breheny wrote:
> Hi Rolf,
>
> Could you either post or send to me directly your schematic? It sounds
> like what you are trying to do should work properly. I've never used
> the MCP6044 but I took a quick look at the datasheet and it looks
> fine.
>
> For future reference, my favorite op-amp for general purpose stuff is
> the LMC648x (6482 is a dual, 6484 is a quad) from National. It is not
> cheap (about $5) so it is not a good choice for most inexpensive
> commercial applications, but for hobbyist one-offs it can't be beat.
> It is rail to rail input and output, ultra-low input bias current, max
> 1mV input offset voltage, pretty low noise, 1 MHz gain-bandwidth
> product. The MCP6044 seems to trade off GBW product for supply current
> (it has about 1000 times less supply current than the LMC6482 but also
> almost 100 times slower).
>
> Sean
>
>
> On 5/5/07, Rolf <learr@rogers.com> wrote:
>   


The top is a simple voltage follower. In my case I am getting erroneous 
output at low input voltages. Specifically, the charger is currently 
charging at 12.5mA as measured with one DMM. Another DMM measures the 
output of the charging chip at 12.7mV (pretty close...). The same DMM 
that measures 12.7mV on the charge chip output (input to the follower), 
reads 1.5mV on the op-amp output. It should be 12.7!!! If I put a load 
on the op-amo out (255R to ground), the opamp out measures 2.1mV. Still 
very wrong.

In the second circuit, I tried to set up a "difference" circuit. The 
theory is that the output should be (4.5V - input) * (100K/200K). Or, an 
"Amplified difference" with a 2x gain. I was figuring I could correct 
the inversion using math on the A/D module.

The problem with the second solution is that the output of the opamp is 
'pushing' back against the input signal. Currently the charge signal is 
12.3mV (I type slowly, it has charged a little more). But, when I 
connect it to the op-amp circuit, it rises to 20.2mV, and the out put of 
the opamp, which I expect to be about 24.6mV less than 4.5V, is actually 
2.36V. So, I am confused ;-)

 From the charger's datasheet they make it clear not to load the signal 
in any way.... and the second circuit obviously messes with it.

My original circuit was circuit 3. A simple gain amplifier with gain of 
2. This woks fine for larger values of the input signal.

I am investigating other options and Jinx's negative rail (but I will 
have to use a different op-amp I think).

Rolf
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