I'm using a DAC to generate a 16-bit voltage supply, using a current buffer (unity gain amp) to enable high current draw and protect the DAC output, and then sensing the current delivered to a load using a current sense amp. Please see schematic.

With an open circuit load (should draw no current) the buffer seems to be pulling a lot of current (the solder job is fine, tested on multiple boards).

Is there something wrong with performing the current sensing operation the way I'm doing it? i.e. is there something wrong with this circuit that I'm not understanding?

Some useful information

  • buffers I'm using are unity gain stable

enter image description here

  • \$\begingroup\$ Previous comment moved to answer. \$\endgroup\$ – Robherc KV5ROB Apr 13 '17 at 0:11

I'm not seeing any feedback on the LT6105.

With no feedback, it's acting something like a comparator, rather than a linear amplifier. Is this the intention?

FWIW, "real-world"/non-ideal op-amps have a "DC Offset Voltage" from the two inputs not being perfect mirrors of each other.
In many designs, this is considered negligible as feedback negates much of it, or the input voltages/swings are large enough to mask it.
However, in your circuit, with an open Vout, the inputs should in theory be seeing perfectly identical inputs, so this offset can cause a false reading on the output.

Would adding negative feedback to the LT6105 be a possible option here?

Here's a "spitball" idea that should give the LT6105 about a 100:1 voltage gain for the ADC to sense. Can you let us know if this works for you, or what constraints you have that prevent its use?


simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Oookay...2 downvotes and no comments...anyone care to comment on why they hate my answer? Constructive criticism teaches & improves the site, but downvoting anonymously just lowers your rep & mine together. \$\endgroup\$ – Robherc KV5ROB Apr 13 '17 at 18:51

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