> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Richard Prosser
> Sent: 09 July 2009 22:27
> To: Microcontroller discussion list - Public.
> Subject: [EE:] Cheap Quad opamp. wanted with high Input bias current.
> 
> I'm looking for a cheap quad opamp with the same pinout as the LM324.
> The killer is that I want one with a high input bias current. I'm
> using a LM324 (LM224 actually to get the high temperature capability)
> at the moment as a sensor amplifier and use the voltage rise caused by
> the bias current to detect if the sensor goes open circuit.
> Trouble is that some of the existing opamps have sufficiently low bias
> current to balance the leakage current and don't hit the sensor fail
> threshold. If I use a high speed opamp like the MC33274 it works fine,
> every time but a) costs more and
> b) is so much higher in transition frequency that I'm worried the RF
> immunity will suffer. -  I'd probably have to get the item retested.
> I can add additional pullup resistors but that would mean a pcb change
> and also (probably) retesting. So I'm looking for a cheap quad opamp
> with a transition frequency in the 1 MHz  area with a minimum input
> bias current somewhere around 100nA or higher
> 

I'm not sure you'd ever get a guaranteed minimum input bias current,
it's not a well controlled parameter and not something you should be
relying on for operation of your design.  There are also numerous other
parameters that you haven't specified for your design, e.g. supply
voltage, input offset voltage, rail to rail input/output required?

Could you mount a resistor on the bottom of the board, (either wire
ended or maybe surface mount) to achieve the correct operation without
modifying the PCB?

Regards

Mike

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