Author(s):
Quan Qing - Lieber Research Group, Harvard University
Charles Lieber - Harvard University
A lock-in amplifier is an instrument used to detect very small AC signals in a noisy environment. For example, a 50 mV signal of interest may be surrounded by 5 V of white noise. Scientists use these amplifiers in many areas of research, including medical, biological, geological, and materials research. There are several manufacturers of lock-in amplifiers, and various frequencies, amplitude, and phase measurement ranges are available.
At the Lieber Research Group, which is in the Department of Chemistry at Harvard University, we had an application to detect specific bio-molecules/viruses using nanowire field effect transistor (FET) arrays. The diameter of a nanowire is a scant 10 billionths of a meter (10 nanometers) in diameter, or about five times smaller than a virus. Nanowire FETs are superior to conventional bulk Si FETs in terms of surface charge sensitivity because of their one-dimensional structure and direct versus buried device geometry. Nanowires have potential applications in detecting disease markers in body fluids that are indicative of malignancies, such as breast and ovarian cancers, and other types of diseases, as well as pathogens used in biological warfare. The sensor is so small it may be possible to implant detectors in the body to continuously monitor levels of insulin and other critical molecules. By correlating signals from an array of multiple sensors instead of from a single element, statistical methods can be used to reduce and/or eliminate, noise and false positive signals thereby enabling much more robust disease diagnoses.
To obtain signals simultaneously from an array of nanowire FET sensors, we required multiple channels of lock-in measurement of very small current signals. We used a virtual lock-in amplifier solution with the following capabilities to extract 128 channels of signal:
- Simultaneous acquisition rate of 25 kS/s (minimum)
- Lock-in amplification at a minimum of ten update per second
- Ability to extract a signal embedded in noise comparable in amplitude
- Ability to stream 32 channels of data to disk at 102.4 kHz
Using the National Instruments Lock-In Amplifier Start-Up Kit, we found an example of a virtual instrument (VI) that could be modified and expanded to meet our needs. The example provided in the start-up kit included an algorithm written in NI LabVIEW that handles the phase-locked loop, filtering, and demodulation steps necessary to perform a lock-in measurement.
We needed to address two issues in order to meet the requirements of this application. First, software-based lock-in techniques are computationally expensive. This required consideration of the code that could optimize processing power for a multi-channel application. We determined we needed to use a dual-processor system and distributed computation.
Second, we had to determine the necessary specifications and requirements and consider if a VI could effectively replace its traditional hardware-based counterpart.
We proposed a multichassis solution to support a 128-channel lock-in amplifier requiring 25 kS/s and a one-second update rate. The system would include four chassis, each with four dynamic signal acquisition (DSA) modules, an NI PXI-6653 timing and multichassis synchronization module to synchronize the chassis, and a single-processor PC to control each chassis.
The original lock-in VI was intended for two input channels: the signal of interest and a reference frequency. We modified the LabVIEW code to accept multiple input channels and made additional changes in order to use two distinct parallel processes for acquisition and analysis. To handle data streaming, we selected a distributed PC architecture. The PXI chassis were connected via MXI to their own PC for streaming data to disk. The research team synchronized acquisition between the chassis using the NI PXI-6653.
The measurement requirements specified current measurement - a 25 kS/s simultaneous sampling rate on all channels - as well as the ability to extract stable signal from comparable ambient noise. The NI PXI-4472 was chosen for its 102.4 kS/s simultaneous sampling rate, as well as its anti-aliasing filters and 110 dB dynamic range. Custom gain amplifiers between the nanowire specimen and the PXI system handle the conversion from current to voltage.
The virtual lock-in amplifier system we developed is currently in use by the Lieber Research Group. As the chart below shows, the system provides improved cost, smaller dimensions, and more flexibility than its traditional counterpart.
Price
$93,100 USD - Virtual
$512,000 USD - Traditional
Dimensions
[275 by 177 by 396.5 mm] x 4 - Virtual
[495.3 by 133.5 by 431.8 mm] x 128 - Traditional
Flexibility
Yes - Virtual
No - Traditional
For more information, contact:
Quan Qing
Lieber Research Group
Harvard University
E-mail: quanqing@cmliris.harvard.edu
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