Bode Plots with Digilent Analog Discovery

Oct 22, 2012

The Analog Discovery design kit along with the free Waveforms software is intended as a modern, powerful, portable, affordable and hands-on approach to teaching electronic circuits in schools. The kit is also a great choice for hobbyists, scientists, musicians and DIYers. Although the kit is primarily intended as a learning tool for building and measuring circuits on breadboards, Discovery can also be effectively used to test and measure existing equipment up to about 10 MHz. The example below demonstrates using the Bode plot capability of the Waveforms software along with the Discovery unit.

Bandwidth of Audio Equipment

High gain audio equipment, such as the M-Audio DMP3 microphone preamplifier demonstrated below, can be measured. However, the high gain can introduce noise in the Bode frequency response measurement since the WG (Waveform Generator) and Channel 1 of the Discovery unit are typically connected in parallel at the input of the high-gain stage in order to provide a response curve relative to the "filter input" amplitude. In this configuration, depending on the gain of the audio equipment to be tested, the WG level must be set very low in the 20 mV or less region. Along with the differential input of Channel at the input, residual noise can be introduced from the Discovery unit. A simple solution to this is to introduce a simple resistive voltage divider network between the output of the Discovery WG and the input of the gear to be measured. This allows the WG signal level to be raised to the 1V level, and tends to isolate the WG and Ch 1 from the high-gain audio gear, improving the noise considerably. Of course the gain in dB of the Bode plot is scaled down by the attenuation value. The example below shows a typical configuration. The attenuator box is a simple home made switchable resistive divider. In this measurement the M-Audio DMP3 preamplifier is set at a gain of about 100 (40 dB) and the attenuator is set at -40 dB. The WG output can then be set at 1.4Vpeak (or 1.0Vrms) providing an input level to the DMP3 of 14mVpeak and a nominal DMP3 output level of 1.4Vpeak. The images below show the configuration and detail of the input connections. The 6 pin headers provided with Discovery facilitate making the parallel input connection using a breadboard. Simple RCA jack adapters can be used to connect with the Discovery leads. A short homemade BNC adapter is shown (Pomona 3778 BNC (F) with isolated ground).

The Bode plot for the DMP3 below exhibits an exceptionally flat response from 10 Hz to 300 kHz with a f3db bandwidth of ~ 550 kHz :

Bandwidth Measurement to 10 MHz

Although the Discovery is specified as having an analog bandwidth of 5 MHz, AC measurements of the Discovery scope inputs show a response at 5 MHz down by only ~ 0.05dB and -1 dB at 10 MHz. Since the sampling frequency is 100 Msps, this means that useful Bode response measurements are possible up to 10 MHz. The example below shows the Bode plot for a simple non-inverting voltage amplifier. Again, to minimize noise introduced by the Discovery WG and CH1, an resistive attenuator was used. The op-amp for this measurement has a GBW of 500 MHz and with the components shown, the expected f3db bandwidth is 10 MHz. The measured Bode plot shows a f3db of just over 10 MHz. The RC high-pass input filter has a 1.6kHz f3db point:

Reducing the feedback resistor of the circuit above to increase the design circuit bandwidth to 25 MHz produces the Bode plot below, flat to 10 MHz. Also the input capacitor was increased to lower the input high-pass filter corner to ~ 150 Hz: