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Old 16th September 2007, 06:18 PM   #1
VEC7OR is offline VEC7OR  Lithuania
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Default Buffered Passive I/V vs. Active I/V

Can somebody explain which is better and why ?

Lets say we have some current source dumping its current into a resistor which is then followed by non-inverting opamp amplifier.

In other case we have classic I/V with inverting opamp and current being dumped into virtual ground node.

The thing I'm dealing with is not quite audio, but it should make good analogy as with current output DAC with opamp.

It well known that BJT input opamps have lower voltage noise, and FET input have lower current noise, so IV conversion is better done with FET ops, but on other hand BJT ones have better figures overall, so my point is that it is possible to get better noise figure with passive IV with BJT opamp than with classic active IV with FET opamp.

Another thing - source has neglible capacitance if that matters.

Frankly I'm out of options what to google and where to ask.
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Old 18th September 2007, 12:23 PM   #2
oshifis is offline oshifis  Hungary
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You could be more specific, what are your design goals. Lowest noise, lowest distortion?
Two things come to my mind: one is the voltage compliance of the current output. In passive I/V there is a voltage drop on the resistor that may influence the current output linearity.
In active I/V the opamp should be able to supply the same current into the feedback resistor as the current source. It is like you loaded the opamp output with the feedback resistor. The opamp may distort if loaded with a low value resistor.
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Old 18th September 2007, 04:21 PM   #3
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Hi VEC7OR,

Many DAC don't accept passive I/V conversion. To keep low distortion, keep output DAC voltage to zero volt. Generaly, DAC voltage must not exceed 20mV.

What kind of DAC do you want to use?

Eric
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Old 23rd September 2007, 12:38 AM   #4
VEC7OR is offline VEC7OR  Lithuania
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Thanks for replies guys, I've already settled with active I/V, I've tried passive version on prototype and it wasnt that good.
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