Instruments4Chem

High Performance, Low Cost…

Propeller Lock-In Amplifier-Based Optical Detection System (LIA-ODS)


The LIA-ODS is a lock-in amplifier-based optical detection system that is ideal for fluorescence monitoring of analytes such as dissolved oxygen, chlorophyll, riboflavin and quinine. Other potential applications include measurement of weak IR absorptions and nephelometric turbidity.

Unlike detection systems described elsewhere on this website, this particular instrument employs phase sensitive detection (PSD). In a PSD-based instrument, the excitation light source is square wave modulated, and only detector signals registered at the same frequency contribute to the measurement. As a consequence, PSD offers excellent noise immunity and immunity to stray light.

The LIA-ODS PCB is shown in the image below. To use the instrument one simply connects a light source and a detector at mini DIN J1.
The LIA-ODS PCB pictured above provides square wave modulation to drive a light emitting diode (LED). Both voltage and current inputs are available on-board to support a variety of detector types. To develop low cost applications, photodiodes and light-to-voltage (LTV) detectors are an excellent choice. For measurements of analytes at trace levels, a photomultiplier tube (PMT) is a far more sensitive detector and would be the preferred option.

The LIA-ODS is designed for direct compatibility with PMT’s such as the common 1P28. A combination of a 1P28 and Hamamatsu C8891, that contains a high voltage supply integral to the base is an excellent combination giving an extremely compact detector package. The LIA-ODS has an on-board regulated power supply and a digital-to-analog converter (DAC) to set the PMT bias voltage under program control, allowing direct control of the PMT gain without any additional circuitry.

The signal from the user’s chosen detector undergoes further amplification with programmable gain before passing to an Analog Devices AD630 lock-in amplifier chip (LIA), allowing recovery of a DC signal that is digitised by an on-board analog-to-digital converter (ADC). Data is passed back from the LIA-ODS to a host PC running a LabVIEWTM control/data acquisition vi via a USB interface.

The LIA-ODS LabVIEWTM vi permits user control of the following parameters :

Signal sources – Current (photodiode or photomultiplier) and voltage (light-to-voltage converter)
PMT bias voltage (can be set in the range 0 to -1000V)
Current amplifier gain (from 1 μA/V to 3300 μA/V in 8 steps)
Instrumentation amplifier gain (from 6x to ~ 1000x in 256 steps)

LIA-ODS Block Diagram

Stacks Image 5012

The Propeller LIA-ODS, featuring a modulated LED source and a phase-sensitive detection system.

Technical Description


The core of the instrument, shown in block diagram form above, consists of a P8X32A-Q44 Propeller chip, a 24LC256 serial EEPROM, and an FTDI FT232R USB-to-TTL interface chip.

A single I/O pin on the Propeller provides a software DAC that drives an RC filter and op-amp buffer (1/2 TL082), generating a PMT bias control signal. This controls a commercial PMT high voltage bias supply with a gain of -1000 (i.e. 1V drive corresponds to –1000V bias). This signal is brought out on the mini-DIN connector at J1 pin 3.

Two types of optical detectors can be connected to the LIA-ODS PCB. A current output signal from an optional PMT detector entering at J1, pin 5 connects to an LF356 trans-impedance amplifier whose feedback resistor is switched in by a DG508 analog switch. The current-to-voltage conversion gain of this amplifier is set by a 3 bit control word from the Propeller chip, thereby providing 8 different gain settings. The output of this current-to-voltage conversion stage connects to one input of a DG419 analog switch.

A second input to the DG419 comes from a TSL25x light-to-voltage converter chip (AMS) that would be connected at J1, pin 4 – this is an alternative yet very sensitive, low cost, solid-state detector. Another digital I/O line from the Propeller allows the user to select which of these two optical input signal sources is being measured.

The analog output from the DG419 is capacitively coupled to an AD623 instrumentation amplifier whose gain is set by a MCP41010 digital potentiometer. A 3 wire serial interface to the Propeller allows the user to set the resistance of this potentiometer, giving this processing block a voltage gain of 6-200.

The output of the instrumentation amplifier then passes to the input pin of an Analog Devices AD630 lock-in amplifier, whose modulation input is driven by a square wave generated on another pin from the P8X32A (and whose frequency can be changed under user control). The same square wave is brought out on mini DIN connector J1, pin 2, so that the output of the user’s source LED can be similarly modulated.

The output of the AD630 undergoes low pass filtering and buffering (1/2 TL082) before passing to an LTC2440 ADC, the interface to which uses a further 5 Propeller I/O pins. An ADR441 provides a stable 2.5V reference for the ADC.

The instrument is controlled by a LabVIEWTM vi. The user simply selects the desired signal source (PMT or TSL25x), the PMT bias voltage and current gain (if applicable) and the instrumentation amplifier gain and the system then measures and logs the measured optical signal in real time.

LIA-ODS J1 mini-DIN6 Pinout

A low cost measurement solution could combine an LIA-ODS with a versatile optical detection system, or VODS. In this case the LED source board would use the modulation signal output on pin 2 of mini-DIN J1 to flash the LED at the desired frequency. This in turn would result in modulated fluorescence that would be processed by the AD630 LIA block.
LIA-ODS mini-DIN pin#
Function
1
GND
2
LED modulation
3
PMT -HV bias
4
Current input
5
Voltage input
6
+12V