To sum it up: I am using the MC34071A driver with a 1K load resistance, and using the 1N5822 (Shottky) diode to raise the ground reference +0.3V. This gives me a negative 0.3V supply for the op amp. The output voltage is Vin x 10 + 2mV, over the entire range. The +2mV error is perfectly acceptable in this application, even at the low end, since I only needed to be able to tell the difference between 0 and 7mA (0V and 1.75mV). Thank you very much for your help. Best regards, Vitaliy ----- Original Message ----- From: "Spehro Pefhany" To: "Microcontroller discussion list - Public." Sent: Monday, November 21, 2005 4:19 PM Subject: Re: [EE] Current sensing - op amps > At 11:48 AM 11/22/2005 +1300, you wrote: >>>>You are hitting up against the output voltage swing specification. The >>>>low >>>>side swing (VOL) of the MCMC34071A is 0.1V Typical, 0.3V Maximum with a >>>>2K >>>>load on the output. You are seeing 0.18V which is in spec but towards >>>>the >>>>maximum side. If your output load resistance is greater than 2K then >>>>lowering that to 2K will likely get you closer to the typical value of >>>>0.1V. >> >>If you can add a "whisper" of negative supply to opamps like this then you >>can swing to true ground. Even a few tenths of a volt will do. This can be >>accomplished with eg a simple diode pump driven from just about any >>available signal. >> >>Another "naughty" but potentially (pun vaguely intended) trick is to place >>a Schottky diode in the ground lead of the power supply. Decouple this >>well (large cap across the diode and also a small ceramic). Take negative >>opamp supply rail from diode cathode and system ground from diode anode. >>This gives you a Schottky diode voltage drop (typically a few tenths of a >>volt) as the opamp negative supply. Voila - true ground output. This >>method has its potential "problems" but may well be perfect in your >>application. This of course reduces the general supply rail (Vdd-Vss) by >>the diode drop. eg from 5v to 4.8v or whatever. depending on your >>regulator you may be able to increase it by the same amount (regulator >>divider change with eg LM317) or add another Schottky in the regulator >>ground return (care needed) OR the reduced supply may still be in spec and >>acceptable. This is a method that must be applied with understanding BUT >>can produce magical results. (Any technology ...). (An 'ordinary' silicon >>diode will also work but produces a much greater voltage drop). -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist