I am having trouble with a design which uses either the LMC6001 or the LMC6081 and I have a suspicion that the magnitude of the problem may relate to the value of the open loop gain.I have questions about the gain figure quoted in both the LMC6001 & LMC6081 data sheets.
The LMC6081 datasheet gives a headline voltage gain figure of 130dB in the "Features" section of the front page. The detailed specification gives a number of figures for large signal gain under different conditions, but the highest of these is 1400 V/mV; equivalent to 123dB. Looking at figure 8, I guess the discrepency is because the gain carries on increasing as the load impedance rises above 2k, but under what condition is the 130dB figure appropriate?
Both data sheets give a significantly higher gain when sourcing current than when sinking current. The difference is around a factor of 4. The notes state that the tests were carried out with a single supply configuration (15V) with the "sourcing current" figure applying when Vout > 7.5V and "Sinking current" when Vout < 7.5V. In my circuit, the output is used to drive a PNP transistor gain stage, so it is always sinking current whatever the output voltage happens to be. Should I then apply the current sinking gain figure even when the output voltage is > 7.5V? Could I get a higher open loop gain if I included a high value dummy load down to 0V so that the output was always sourcing current? Is this change in open loop gain depending on the direction of current flow reflected in the SPICE models?
