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Precision Sine Wave Generator

John Ramshur
Prodigy 50 points


I need help designing a precision sine wave function generator for calibrating a bio-potential amplifier. I need a 100 uV, 1 kHz sine wave with very low noise and low distortion.

I've tried using a BK Precision function generator and a voltage divider with standard 1% metal film resistors, but the SNR is horrible. I know there are several basic oscillator circuits out there, but I don't have any experience in designing precision instruments. So far all I know to do is purchase precision resistors and op amps to reduce noise.

  • 0
    Expert 2420 points

    It's not my personal working topic, but I've got some questions and ideas:

    1. You just used a resistive divider to attenuate to 100 uV?
    2. Which output impedance do you need?
    3. I'd suggest to try reducing the bandwidth by adding a capacitor (foil), hence I would try a resistive divider wth additional output capacitor:

    sinus output (140 mV, 1 kHz) o-------R1-----o-----o----o output
                                                                              |       |
                                                                              |       |
                                                                             R2     C1
                                                                              |       |
                                                                           GND GND


    R1=10 kOhm, R2=1 kOhm => ratio=1/11
    output resistance: R1||R2=10/11=909 Ohm
    C1=2.2 uF, foil => low pass filter with fc=80 Hz => 1 kHz is attenuated by 12.5 => with the attenuation of 11 of the resistive divider the 140 mV at the generator output you should make 100 uV at your output.


    Best regards and good luck,
    Edwin Krasser

  • 0 in reply to Edwin Krasser
    Guru 21975 points

    John, Edwin,

    I think a simple voltage divider should be able to accomplish this function. For example, a 100k / 10-ohm resistor divider will divide a 1V signal down to 100uV.. The noise contributed by the divider is the noise of the parallel combination, essentially 10 ohms. The noise produced by this resistance is very low and would be very difficult to detect with any commonly available amplifier. The 10-ohm output impedance of the divider is unlikely to need buffering with another amplifier.

    If this is what you have already tried, the noise is probably creeping into the circuit in some other way.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 50 points

    What would you guys suggest for a portable, battery powered precision sine wave generate circuit and components?

  • 0 in reply to John Ramshur
    Expert 2420 points

    * Wien bridge oscillator
    * a small MSP430 + Crystal + DAC (with sufficient resolution) + AAF as the more flexible and complex solution

    Best regards,
    Edwin Krasser