Original question:
Hello,
I want to generate 100ma ac signal current. (output swing voltage will swing on a 1.65V dc reference voltage)
I tried to do that using improved howland current source circuit with opa551 on tina but i cant get right conculusion.
Hello Mustafa,
Below, see an example of an Improved Howland Current Pump using the OPA551 power op amp. I have set it up for a 100 mA peak output. Do keep in mind that the load resistance applied to the output must not require an output voltage that violates the op amp's output voltage compliance range. Operating the OPA551 with +/-30 V supplies allows a load up to about 250 Ohms to be used.
If you want to offset the output by +1.5 V, then a small voltage can be added in series with the R5 input resistor connected to the non-inverting input. For the component values I selected for the pump it takes only about +58 mV at that point in the circuit to shift the output +1.5 V. Different component values will have produce a different offset voltage.
I have included my TINA circuit for you evaluation.
OPA551_OneOpAmp_Howland_01.TSC
Regards, Thomas
Precision Amplifiers Applications Engineering
Hello Mustafa,
When I review your OPA547 Improved Howland Current Pump schematic there is a problem with the implementation. The OPA547 minimum supply voltage is specified as +8 V, and your TINA schematic shows +3.3 V. Even though the OPA547 model may function with a +3.3 V supply, the real device won't. Also note that the OPA547 output does not swing to 0 V in the negative direction when a single +V supply is used. The closest it can swing to 0 V will depend on the current the output current the output must sink/source while remaining in the linear output operating region. Therefore, the OPA547 should be operated with a dual, plus/minus supply.
I do think a CMOS, power op amp such as the OPA569 would be a better choice for your application. It is a low-voltage, rail-to-rail input/output device that is better suited for your application shown in the TINA file. See the datasheet here:
http://www.ti.com/lit/ds/symlink/opa569.pdf
I've copied a couple of slides from some TI Precision Amps training materials that should help you with designing an Improved Howland Current Pump. I think this will help you get to answers more quickly.
Regards, Thomas
Precision Amplifiers Applications Engineering
Hi Mustafa,
I'm still not clear what you are trying to do. But if the load is only varying so little, you might not need the Howland Current Pump. The circuit below will also provide a nearly "constant" current. The deviation is only 0.17%:
Kai
Hello Mustafa,
Your calculations are correct. It is your supply implementation that is causing problems with the Improved Howland Current Pump operation.
Powering the OPA551 with a single +12 V supply and applying a 0 V to +3.3 V input waveform violates the lower VCM Common-mode voltage input range of (V–) + 2.5 V. There is a similar issue with an linear output voltage range violation, where VOUT Voltage output of (V–) + 3 V minimum.
The solution to getting your OPA551 working is to add a minus supply such as -5 V to the V- pin. You can see a TINA simulation comparison between the a +12 V single supply and +12 V/-5 V dual supplies scenarios here:
Due to component tolerances and OPA551 specification variances some minor adjustment of the series output resistor will be necessary to get the current to the exact level you need.
I hope this information helps get your design moving in the right direction.
Regards, Thomas
Precision Amplifiers Applications Engineering
