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Hi,
I'm using OPA547 as V to I converter with +/-15Vdc supply and a maximum current of +/- 50mA. I use 4 IC on the board in order to implement 4 V/I converter. The source is made by the 4 channel DAC8734 with the same power supply.
The output current formula is: I = -Vdac/200 and is independent form the load.
I set the current limit to approximately 55mA by adding a 390k resistor between pin 3 and 4 with a 10nF capacitor in parallel.
I tested the output current with a 100ohm load and I see on 2 of 16 boards that 1 IC limit current is below 55mA: on one board is 38mA (3,8V on the load) and on the other is 47mA (4,7V). The issue is on 2 different V/I converter
I try to solder the OPA547 but the issue remain.
The DAC output is about -10V as I expect.
In order to improve the current limit until 50mA I had to put a 120k resistor which means about 160mA current limit!
On datasheet I see a tolerance of +/- 30mA in current limit value for a load of 375mA.
Here are my questions:
1) the +/- 30mA tolerance is relative or absolute to the output current? Even if is absolute I don't understand why I need 160mA theroetical value to get 50mA output. I also try to chance the component but I have the same result (maybe also this has the problem)
2) How can I fix this problem?
Thanks
Hi Davide,
How can I fix this problem?
Per your description, I think that you have Howland current pump configuration with OPA547, see the circuit below.
I think that issues may not have something to do with ±30mA tolerance specification (the application did not reach I_lim threshold). The current source is regulated by R3 or 200Ω and OPA547's Vout voltage.
With Rload = 100Ω load, Vload = 5V. Since the OPA547's supply voltages are configured at ±15Vdc, the power amplifier's output voltage reached a saturation and unable to go higher, where Vout = Vfb + Vload for Iout = 50mA. The datasheet is specified Io = ±100mA, but your ±50mA have the similar Vout swing limits.
To get around the issues, you may have to increase your supply rails slightly. Or lower your Rload below 100Ω.
Enclosed is a Howland Current Pump app note for your reference. Note that I did not buffer the OPA547's feedback and I have to use large feedback resistors to reduce the current errors in the simulation.
If you have additional questions, please let me know.
Best,
Raymond
Hi Davide,
you shot show your schematic. Without schematic we are blind and have to guess 
Kai
