I’ve been working with the ADALM1000 for a few months now and I thought it might be a good time to dust off this idea I wrote about in the November 2011 issue of Analog Dialogue and see how well it works with the ALM1000.
The DC coupled inputs of the ALM1000 have a distinct advantage over the AC coupled inputs of a PC sound card.
The limited bandwidth of the ALM1000 can be extended, for repetitive waveforms, by using a sampling front end ahead of the analog inputs. Sub-sampling the input waveform with a high-speed sample-and-hold amplifier (SHA), followed by a low-pass filter to reconstruct and smooth the waveform, effectively stretches the time axis, allowing the ALM1000 to be used as a high-speed sampling Oscilloscope or Spectrum Analyzer.
The AD783 SHA used in this sampling font-end operates from +/- 5 Volt power supplies. The input/output swing with ±5 V supplies is at least ±3 V and this will need to be scaled and offset to better interface with the 0 to 5 V input range of the ALM1000. This was done with a resistor divider as shown in figure 1. By connecting R2 to the +5 V reference we introduce an offset such that a +/- 3 swing at the op-amp becomes a +1 to + 4 swing centered on 2.5 V at CA-In. Relatively low value resistors were chosen to avoid frequency response roll-off from the large 400 pF input capacitance of the ALM1000. The op-amp output of the sampling circuit will have no problem driving this resistance.
Figure 1, Resistor divider scales signal amplitude and offset to interface to ALM1000
The arbitrary waveform generator outputs from the Analog Discovery were used to test the combination of the AD783 sampler and the ALM1000. AWG 1 was configured to produce a +/- 1 V two tone waveform at 3 MHz and AWG 2 was configured to produce a +/- 1 V single tone waveform at 2 MHz. The sampling clock of the AD783 was adjusted such that the 2 MHz sine wave was down converted to 500 Hz. A ratio of 4000.
Figure 2 AD783 SHA measuring Analog Discovery – time domain plot
The time scale is stretched by a factor of 4000 such that the 2 mSec/div becomes 500 nSec/div in the ALICE screen shot time domain plot shown in figure 2. In figure 3 we have a Waveforms screen shot of Analog Discovery measuring itself with the same vertical and horizontal scales. As we see the two results are indistinguishable.
Figure 3 Analog Discovery measuring itself – time domain plot
Figure 4 is an ALICE-SA frequency domain plot of these test waveforms. Again the horizontal frequency range is scaled by the same factor of 4000 such that the 0 to 5000 Hz becomes 0 to 20 MHz. In figure 5 we again have a Waveforms screen shot of Analog Discovery measuring itself with the same vertical and horizontal scales.
Figure 4 AD783 SHA measuring Analog Discovery – spectrum plot
The single tone traces in both cases are very similar with the AD783 sampler having somewhat worse second harmonic than seen in the Analog Discovery trace. The rest of the harmonics are about the same with the noise floor actually a little better in the AD783.
The two tone traces show the exact same intermodulation components at 16 MHz with the same amplitude level. The other spur levels are similar but occur at different frequencies. The noise floor is a little worse for the AD783 sampler.
Figure 5 Analog Discovery measuring itself – spectrum plot
The sample clock circuit proposed in the Analog Dialogue article is cumbersome to adjust and tune. It was a workable solution for use with a PC sound card where there was no ready access to digital outputs to software tune the sampling clock. The ALM1000 has 4 digital I/O pins and these could be used to control a DDS clock source over SPI such as the AD5932. Stay tuned for more on that in coming months.
In conclusion we can use a high input bandwidth sampler to increase the bandwidth and effective sample rate of the ALM1000 to the equivalent of the Analog Discovery.
As always I welcome comments and suggestions from the user community out there.