Many lock-in applications use a drive signal consisting of a small AC excitation voltage and larger DC offset voltage, for example differential conductance (dI/dV) measurements. Users of the MFLI Lock-in Amplifier have the choice between two methods to generate such a signal: using the analog adder ("Add"), or using digital offset generation ("Signal Output Offset"). Figure 1 shows where you can find these functionalities inside the MFLI's user interface LabOne. The digital method is a good solution for small, static offsets. For large and/or varying offsets, the analog adder method offers two performance advantages. First, it allows you to use a smaller output range, giving you better amplitude resolution for the AC component. Second, it leads to an AC amplitude that is more accurate and insensitive to changes of the DC offset. In this blog post, I demonstrate how to use the analog adder, show its performance in a measurement, and briefly discuss the technical background.
Figure 1: Controls for the analog adder and digital offset functionality in the MFLI user interface LabOne. The Signal Outputs section shown here is located in the Lock-in tab.
Figure 2 shows that upon activating the "Add" button, the DC voltage applied to Aux Input 1 is added to the AC signal after the output range stage. The DC voltage can be as much as ±10 V independently of the selected output range. In analog adder mode the D/A converter (DAC) typically generates a zero-offset signal. In comparison, when generating the offset voltage digitally using the Signal Output Offset parameter, the DAC has to generate the AC component at a different working point inside its range. This means that real-world DAC imperfections such as nonlinearity can become visible as a change in AC amplitude. This is the reason for the better amplitude accuracy achieved with the analog adder.
Figure 2: Circuit diagram of the MFLI output stage showing the location of the analog adder.
The difference in accuracy between the analog and digital method is most clearly observed in a sweep of the offset voltage. We use the setup shown in Figure 3 to do such a measurement. Signal Output +V is connected to Signal Input +V to perform a lock-in measurement. In order to supply the DC analog adder voltage, we connect Aux Output 1 to Aux Input 1.
Figure 3: Wiring used for the sweep of the analog adder voltage.
We generate an alternating signal of amplitude 10 mVrms on Signal Output +V and measure this signal on Signal Input +V using AC coupling. We then configure the Sweeper tool in order to sweep the offset voltage from 0.0 V to 0.9 V. The results are shown in the following figure in which we plot the measured signal amplitude (R).
- The blue curve shows a sweep of the analog adder voltage. For this measurement, ensure that the Aux Output 1 Signal is set to "Manual" in the Auxiliary tab. Enable "Add" and select Aux 1 Offset as the sweep parameter.
- The orange curve shows a sweep of the digital offset voltage. For this measurement, disable "Add" and select Signal Output Offset as the sweep parameter.
Have a look at the tutorials called "Simple Loop" and "Sweeper" in the MFLI User Manual for more detailed instructions for setting up such a measurement.
Figure 4: Comparison of the two methods for generating an offset voltage in the Sweeper, analog adder and digital offset voltage.
The blue sweep in Figure 4 indeed shows a flat curve indicating a perfectly constant AC amplitude. There are only small, random variations from one repetition to the next (cf. grey curves) due to noise. The orange curve in comparison exhibits variations that are, unlike noise, reproducible and stable in time. With a size of about 10 μV (in the 1 V range!) the features are small, but for high-precision applications they can matter, so it's good to be aware of them.
Let me conclude this blog post by listing two take-home messages:
- The default way of generating a DC offset voltage on the MFLI is by using the Signal Output Offset parameter which generates the voltage digitally. It's simple and quick, and a good solution for generating a static offset.
- The alternative way, the analog adder, requires one additional BNC cable. This method offers superior accuracy in cases where the offset is swept.
A note to HF2LI users: Your instrument features an analog adder, but the digital offset is not available.
A note to UHFLI users: On your instrument there is no analog adder, only digital offset can be added to the output signal. However, analog signal addition is most easily implemented using an external bias tee and one of the auxiliary outputs.