On Mon, 19 Jan 1998 16:46:04 -0500 Sean Breheny writes: >Hello all, [...] >I want to clarify an earlier question that I asked: > >Does anyone know of an amplifier (op-amp style) with something >approximating these specs? > >* Less than 10nA input bias current You can get around the bias current limitation by providing another path for the bias current to flow rather than through the input terminating resistor. Put another large resistor at the input connected to an adjustable voltage and adjust the voltage until the output voltage is zero both with the input open and with it shorted. The input resistance is then approximately the two resistors in parallel (one to supply, one to ground). This is a good idea even if the amplifier you use has a rather low bias current, as it prevents current from flowing out of the scope and into the circuit under test. >* OK to operate off +5 and gnd (not necessarily input range incl. GND, >just >doesn't need more than 5v spread from Vcc to Vee) You may need a negative voltage to supply the bias current, but this is a good idea anyway since using a dual-supply op amp opens up a lot more choices. Without a negative supply to the front-end, I'm not sure how you intend to be able to measure negative input voltages without lifting the input ground from the rest of the circuit ground (a rather bad idea). A real simple way to get a few mA of bipolar supplies would be too use the voltae converters in a MAX232 style chip. Linear regulators after the converter would be a good idea for consistent performance and to help remove switching noise. >* Less than 10mV input offset voltage Don't get too hung up on maintaining a zero DC offset through the amplifier, it makes it rather complicated. Many, many useful scopes aren't designed to measure absolute DC input voltage (i.e. the VERTICAL POSITION dial isn't calibrated). Digital scopes are lousy as voltmeters anyway due to the small number of bits in the converter. >* Voltage feedback If you're configuring the amplifier as a noninverting amp with the signal applied to the + input and feedback at the - input, a current feedback amp will work fine. They just have a low impedance at the - input terminal, requiring the use of smaller resistors in the feedback network. With proper feedback maintained, the + input terminal is voltage sensitive and high impedance. >If not, does anyone know of a buffer that can at least provide a flat >gain >of 1 and has the rest of these characteristics? I'd be tempted to try JFET source followers between the input and the voltage gain stage. DC offset may go out the window, but my opinion is that that isn't too critical in a scope. >the op amp that I am using.) I have not finished soldering in the >components, though, so I could switch op-amps if there was one which >was >pin-compatible. I may be suggesting a total re-design, but you should explore your present design some more to find out its limitations. The first unit built shouldn't be expected to work really well. >Also, does anyone know of a way to some how cancel out >input bias current without having to know the source impedance? I touched on this above. If the amplifier only has to handle AC signals, then the bias curent can be automatically cancelled. Use another low-performance op amp to supply slow feedback to supply the proper bias current to force the main amplifier's average output to zero. I think this is called a "DC Servo". If you need to amplify signals near DC (which most useful scopes do), then better hope the bias current is constant and use a fixed adjustment. An off-the wall idea would be to use a dual amp, one side connected just to measure and amplify bias current. This would be applied to the other one (which is amplifying the signal) to cancel its bias current. The two amplifiers in a dual chip should track each other fairly close.