Tool/software: TINA-TI or Spice Models
You may find my design. I used 4 parallel op-amp for higher current supply. There is no problem with TINA simulations.
my plan is changing current range with relays. Resistor values may change to compensate current range. I'm also planning to measure applied current to compensate leakages.
Paralleling 4 op-amps will reduce noise or not?
Is this possible to get %1 accuracy?
Any suggestion to improve my design?
How will opa209 output leakage current affect my precision?
Also, I want to use triax connector for current output(R_LOAD). How can I drive inner shield of triax(Guard)?
This uses an OPA1622 as the main amplifier in the current pump. The OPA1622 should be capable of driving 100mA without assistance, and with relatively low voltage noise (2.8nV/√Hz). I dropped the 10kΩ feedback resistors to 1kΩ both to reduce resistor noise and to reduce the impact of U1's input current noise. I swapped an OPA140 (I'd use a dual, the OPA2140, for U2 and U3) for the OPA627, though the OPA627 could certainly be used in this application. U2 serves double-duty in this application, both as the feedback buffer for the current pump, and as the guard driver for the Triax cable. U3 was added as a buffer ahead of the ADC, as low current ranges (especially with high impedance loads) would likely have too low of an output impedance to adequately drive an ADC.
In order to get a good transient response and reasonable stability, I had to add C1-C4 in parallel with the higher range resistors. These values appear to give decent step responses into resistive loads, but I'd double check stability into reactive loads.
You can download the TINA-TI file I used for the transient analysis here
One further note: I connected the voltage source for the input to R8, rather than R6. This shouldn't affect the overall stability of the circuit, only polarity of the output current. If your application needs a negative output current for positive input voltages, simply connect the voltage source to R6 and ground R8.
Precision Amplifier Applications - TI Tucson
In reply to Alexander Davis:
In reply to Koray Alic:
Any type of serial interface is OK.
My goal is %1 resolution in all current range. This current source will perform as a DC. There is no need for high-speed DACs.
I think that using 16-Bit(or higher) DACs within +-5V range might be enough. (DAC8718)
BTW, there are no many options for higher resolutions within +-5V range.
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