[sdiy] the obscure output of a 2164

[rsdio at audiobanshee.com]

The SSM2164 is a current-input, current-output device. Since we nearly always want voltage signals, that current output needs to be converted to a voltage, typically using a resistor to ground.

When analyzing op-amps, remember the two main rules. The op-amp will do everything in its power to make the + and - inputs having the same voltage, but no current will flow in or out of those pins. Thus, the op-amp must adjust the output voltage to make that true.

In the case of the typical SSM2164 buffer, the + input is grounded, meaning that the - input will almost always be at 0 V to match. The positive current flowing out of the SSM2164 cannot flow into the input pin of the op-amp, so it must flow through the feedback resistor. Since there is always a voltage drop across a resistor with current flowing through it, and the input side of the resistor is at 0 V, that means the op-amp output voltage will always be negative (unless the SSM2164 can sink current, in which case the op-amp would produce a positive output voltage).

Thus, the maximum current times the resistance (30k to 300k) gives the maximum voltage.

Brian Willoughby


On Mar 8, 2017, at 10:20 PM, Busby Bergson <busby.bergson at gmail.com> wrote:
> my eyes glaze over a bit with datasheets - and experts have better advice, anyway :
> i'm seeing a lot of designs with the output of a 2164 directly patched to an op-amp....an inverting amplifier arrangement, with a feedback loop, usually somewhere between 30k and 300k..... i have no idea how to calculate the gain of this, though, because i'm unclear on exactly what's coming out of the 2164.. any pointers?
>