Constant (peak) current sine generator

[forHisglory]

Hello,

I would like to hear your suggestions for a circuit to produce a sine wave the has a constant peak current. The frequency would be around 1 Hz, and the positive peak current would be ~ 25 mA (Peak to peak would be ~ 50 mA). The load resistance is not constant.

My first shot at it was to use a DAQ card to produce the sine wave at a given voltage and then I monitored the current with a shunt and then adjusted the peak voltage. It worked pretty well, but I would like a hardware solution.

 

[VEBill]

The old school approach is using a small lamp to provide stable amplitude with perfect sine wave.

I Googled: 'sine wave oscillator lamp' ("lamp" being the magic keyword), and found this very good example:

http://www.bobsdata.com/audio_sine_wave_generator/

You'd need to adapt the output to match your exact requirements.

 

[jimkirk]

Once you have built a stable sine wave voltage source like VE1BLL references, then Google "Howland Current source". This will convert a bipolar voltage to a bipolar current. The output impedance will depend on resistor matching and such, so you need to determine how much error you can tolerate. You might need a booster output stage to get 25 mA, though many modern op amps can do that pretty well.

One other thing you'll have to deal with is the voltage compliance, most op amps like +/- 15 volts maximum, though you can get some that can handle several hundred volts. What is your load impedance?

 

[jimkirk]

Anothe thought; at 1 Hz, a low distortion analog circuit can take many seconds to stabilize. Many small microcontrollers (Atmel, MicroChip, TI) can generate a sine wave in software via a PWM output and a low pass filter. This will also give a much more accurate frequency, easily changed and all that digital goodness. You'll still need to convert a voltage output to a currnt output.

 

[MikeHalloran]

The lamp circuit gives you a sine wave in voltage.

In addition to that, you need a voltage controlled current source, something like this:

http://www.daycounter.com/Circuits/Current-Servo/Current-Servo.phtml

Mike Halloran
Pembroke Pines, FL, USA

 

[forHisglory]

Thanks to all who replied. The load impedance should be in the 50 to 300 ohm ranged, so the voltage compliance shouldn't be too bad.