How to Set up Lock-in Measurements in the Presence of Large Spurious Voltages

September 1, 2022 by Jelena Trbovic

Introduction

Lock-in amplifiers are extremely versatile and used in a plethora of applications ranging from optics and photonics, nanotechnologySPM, and sensing, among others. Achieving high sensitivity of 1 nV in a high noise environment is possible thanks to the phase-sensitive detection working principle of the lock-in amplifier. The MFLI Lock-in Amplifier is designed to accommodate large and small signals with voltage input ranges spanning from 1 mV to 3 V in 7 range steps with 16-bit vertical resolution. This is a wide measurement range suitable for different applications, however, we still need to remove large spurious signals that not only reduce measurement quality but can also damage the input. In this blog, you will learn how to use the MFLI Lock-in Amplifier to carefully assess the experimental conditions and prepare your setup for the best measurement outcome.

Let us start by looking at general measurement considerations.

Your Setup

When the experiment is ready, it is helpful to familiarize yourself with all the equipment and confirm that it performs according to expectations before connecting the sample.

  • In case of the MFLI you can do a simple loop-back from voltage output to voltage input, apply several voltage values and confirm expected results. 
  • Beware, MFLI output can send a maximum of 10 V (AC and DC combined) voltage signals which will cause an overload of the input and a damage.
    • You can prevent this from happening by choosing one of the 4 output voltage ranges 1 mV, 10 mV, 100 mV and 10 V.
  • If you need to use the 10 V range, use a 20 dB attenuator and account for the change when measuring the voltage.
  • Use the LabOne User Interface (UI) to monitor the state of the inputs as shown in Figure 1, the signal side bar tells you what the relative portion of signal with respect to the selected input range. 

When you are sure that each component of your setup is working as you expect, you can connect the instruments to your sample without applying any signals that might damage it, i.e. take all the precaution that your hard work of preparing your sample is not in vain.

MFLI Input

Figure 1. Signal Inputs section of the Lock-in Tab in LabOne UI. Voltage Input is represented on top and the Current Input on the bottom of the section. The Voltage Input signal level indicates the signal amplitude at the maximum level for the 3 mV input range. The Current Input signal level indicates a good choice of the signal and input range ratio, approximately 50 % of the range.  

LabOne User Interface

Since you already have an MFLI, you are in luck as the LabOne UI we provide contains essential tools like Scope, Plotter, Sweeper, etc. With these tools at hand, LabOne is your best friend when setting up the measurements.

  • Connect your MFLI device to your PC and start LabOne user Interface
  • In LabOne, increase the input voltage range to the maximum value of 3 V
  • Make sure that everything is setup properly and connect the BNC cable from experiment’s output into the lock input (e.g. Voltage)
  • Choose the reference frequency in the Lock-in Tab and start by applying minimal AC and/or DC excitations. Zero is also good.
  • Make sure there is no overload indicated on the bottom right with OVI flag. If the flag is red, this means that the voltage you are applying to the input is higher than 3 V. The red LED on the front panel will also indicate an overload. Since you can’t measure under these conditions anyway and only damage the input if the signal exceeds 5 V (see Table 20 Maximum Rating in the MFLI User Manual), remove the cable and make sure you didn’t miss something setting everything up.
  • After further testing and fixing you can turn to the Auxiliary Input which has an input range of 10 V. You can check the actual signal level in the Aux Tab under the Aux Input sub-Tab. If everything looks good i.e., signal amplitude is less than 3 V, you can safely go back to the main Voltage input as it will allow higher measurement sensitivity.
  • If the signal is larger than the 3V, use an attenuator to make sure it fits in the device measurement range

Now that you are confident the signal is under control, and you can use the lock-in safely and you can focus on the signal quality.

The Oscilloscope

Oscilloscopes are essential in measuring the noise spectrum of your experiment. If the noise is large around your signal, this means that your signal-to-noise (SNR) ratio will be small and that you will probably need to measure slower to improve this, unless you can remove the surrounding noise. How to do this?

  • Your lab’s external oscilloscope is always a viable choice, but it is often missing just when you need it. No need to worry: the built in LabOne Scope tool will probe the exact measurement situation with bandwidth of 30 MHz in case of the MFLI. Use the Time and Freq FFT buttons on the right to switch between the time and frequency domain.
  • If your signal contains a large DC component which is much larger than your AC signal and you do not need to measure it, use the AC coupling to completely remove it. You can now reduce the input range and instantly get better measurements. Always make sure that the signal is between 20 – 80 % of the input range whenever possible. Figure 1. shows the example of an overloaded Voltage input and below the signal level of the current input in the right position for optimal measurements.
  • If the signal is much lower than the smallest input range of the instrument, you can use an external pre-amplifier that will place the signal in a better position with respect to the chosen input range.
  • The LabOne Oscilloscope can be further set up to capture any transient and random signals appearing in the network. These you can capture using the trigger of the Oscilloscope as shown below In the Figure 2. with a spurious 900 mV signal appearing while measuring a 10 mV AC signal. The histogram Math function can give you an idea of the signal amplitude distribution.
Capturing Large voltage signal

Figure 2. A snapshot of the LabOne Scope Tool used to trigger on the large DC signals appearing in the network. Such signals can cause clipping of the input and introduce measurement artefacts in higher harmonics and overall noise.

Measurements

When you made sure everything is ready and your signal is free from large spurious voltages you can reduce the input range to such that it encompasses the signal and noise amplitude between 20 – 80 %. The available bits of the input will be used to the fullest and your SNR will be optimal. Further improvements can be made by the setting of the the lock-in filter bandwidth or a time-constant, that ultimately determines the dynamic of your measurements.

Your actual experiment might hide several possibilities for the voltage and environment to change so let us look at a couple of examples.

Transport Measurements

Transport measurements often require applying high voltages that control the state of the device and not necessarily measured. The typical transport measurement setup shown in the Figure 2. requires measuring a small AC signal across the nano-device together with a small DC bias to measure differential conductance spectra. Addition back-gate DC bias is used to change the energy configuration of the device and can be as high as 10’s of volts. The danger is that the gate oxide breaks down and dump the high voltage to the lock-in input and damage it if it exceeds the 5 V mark. To prevent this from happening use a protection circuit that routs the overvoltage to the ground as well as a DC blocker. Leakage currents can be monitored via the current input and the whole setup carefully evaluated using the sweeper tool and the Aux Output of MFLI as a DC source up to 10 V.

Transport

Figure 2. A schematic of the typical 4-point nano-device measurement set up. The gate voltage applied across the gate insulator can exceed the maximum safe voltage that can be applied across the voltage inputs. In case the gate insulator breaks-down, this high voltage is likely to be applied over the voltage or current inputs. 

Optics and Photonics

In most experiments in optics and photonics, the optical signal from the sample is measured by a photodetector whose output voltage is in turn fed to the Signal Input of the MFLI Lock-in amplifier. The photodetectors available on the market have output voltage swings within +/- 3V for a 50 Ohm termination. However, there might be rare cases where larger voltage swings might apply; therefore, to avoid damaging your lock-in amplifier, you need to be cautious:

  • Choose the photodiode carefully:
    • Make sure that the maximum output voltage swing of the photodiode does not exceed +/- 5V for a 50 Ohm termination.
    • When using balanced amplified photodetector, the voltage swings can go up to +/- 10V. If that is the case, make sure the signal is properly attenuated before being fed to the Signal Input of the MFLI
  • If unsure about the actual amplitude of your signal, before connecting it to the Signal Input, check its amplitude:
    • Connect the signal from the photodiode to the Aux Input of the MFLI, as it has a higher rating, and check it using the Scope tab of the MFLI.
DLA

Figure 3. An example of the optics and photonics set up for Tunable Diode Laser Absorption Spectroscopy. The photodetector sends the signal to the MFLI's input. The output specs of these detectors needs to be adjusted to the Input specs of the lock-in amplifier.  

I hope this will help you get started with using the MFLI and give you an opportunity to improve your measurement approach with the ultimate goal to achieve the highest possible  SNR and put you in a position where you can discover new physics.

I'd like to thank my colleagues Gustavo Ciardi and Heidi Potts for helping me, and making the writing of this blog a fun experience!