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 wrote: > Hi all, I have an EE problem that I would like help solving. I have a > Li-Ion charging chip (LTC1733) that, as one of it's outputs, provides a > voltage proportional to the charge current. In essence, in my > application, it is a 1V/A ratio. It will put 1V on the output if it is > charging @ 1A, and 10mV for 10mA charge, etc. The full-range output of > the device is 1.5V indicating the maximum 1.5A charge current. This is > practically a DC signal, there are no bandwidth concerns. > > In addition to the charger, I have a high-precision 4.5V reference > (REF194 from Analog Devices). This is used as the VRef+ of the PIC's A/D > peripheral. The rest of the system runs at 5V > > Currently I am using a MCP6044 op-amp to multiply the signal by 2, to > produce a better full-range 0-3V). I am considering a 3x multiplication > to use the full range of the A/D, but the 3V range is sufficient for now. > > My problem is my lack of experience with op-amps at low input voltages, > combined with a general lack of experience with electronics... I don't > know the right tools for the job... > > The problem: at low signal voltages the OpAmp's limitations cause very > inaccurate readings. It seems that anything less than about 50mV (a > charge current of 50mA) seems to be below the OpAmp's 'low rail'. > Setting it up as a voltage follower I put 10.6mV on the non-inverting > side, I get 4.3mV on the output. Obviously, my DMM may be of dubious > accuracy at this range, but it is consistent. > > I tried to move the signal out of the low-rail area of the opamp by > 'subtracting' it from the 4.5Reference voltage using a 'classic' > difference op-amp layout. But, because the signal impedance is > relatively high, neither summing nor difference mechanisms were able to > get it out of the rail areas of the opamp without impacting the signal > itself. > > Fundamentally the charge currents below 50mV have very substantial > errors. Further, it seems that neither amplification, summing, > difference, not any other op-amp tricks I can find on the web or H7H AoE > can help with this (using the MCP6044 opamp). One option would be to > just ignore anything less than 50mA/50mV as 'trickle' charge, and go > from there, but I do want to try to get this right (it is a hobby, and > learning is a major motivator). > > One solution may be to use a negative voltage on the op-amp, but I > currently do not have one available, and I am reluctant to add one. > Another option may be to use a chip that is more successful in this range. > > So, two questions.... what would be a good way to sample a DC voltage in > the 0-1.5V range from a high impedance source paying particular > attention to the areas between 0 and 50mV? > > Are there better "rail to rail" opamps for this task than the MCP604x > series? (I am using other op-amp units in the 6044 package for other > purposes, not just the charge measurements). > > Thanks for any hints, tips, pointers, and corrections. > > Rolf > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist