TI’s Single Chip Potentiostat
The LMP91000 is marketed by Texas Instruments as a single chip potentiostat solution allowing connection of various 2 and 3 electrode devices, primarily electrochemical-style gas sensors. The chip documentation also discusses applications for the LMP91000 in low cost electrochemical glucose sensing.
The chip therefore looks very attractive for use in low cost instrumentation. Looking at its specifications, one sees that when using a fixed 2.5V reference (Vref), the range of potential control achievable, i.e. |Vwe-Vre| is restricted to approximately 0.6V. Within this range, an internal register controls that potential to discrete values that are fixed percentages of Vref, in 2% intervals up to 24%. The discrete nature of this control and its graininess are not ideal for experiments such as anodic stripping voltammetry where both a larger range and finer potential steps are desirable. The LM91000 does incorporate a current amplifier with programmable gain and this feature is however very useful.
Consequently, experiments are currently underway to operate the LMP91200 using potential control set to 24% of Vref but to provide Vref from a voltage ramp generated by a DAC. This potentially increases the range of |Vwe-Vre| to ~1.2V. The system is being designed as an XMOS Startkit shield, as described below.
The schematic diagram above shows the proposed 4 chip potentiostat system. Pins 12-14 of the LMP91000 connect to the electrochemical cell. Instead of employing a fixed voltage reference at pin 11, this pin is connected to the output of a rail-to-rail AD5660 digital-to-analog converter (DAC), having an output swing of 0 - 5V. Since Vref must not exceed VDD, this necessitates that VDD be set to 5V, and also requires 5V-3.3V logic level shifting circuitry on the LMP91000’s I2C lines, SDA and SCL.
The AD623 instrumentation amplifier (IA) is connected to provide a precision divide-by-2. For this IA, Vout-Vref = Vin(+)-Vin(-). Here, since pin 5 (Vref) is grounded and Vout = Vin(-), Vout = Vin(+)/2. This connects to the - input of an ADC161S626 ADC, while the + input comes from the output of the LMP91000.
As can be seen, ten I/O pins provide the interface to the XMOS Startkit. Two are used for the LM91000 I2C interface, 3 for the ADC control and 3 for the DAC control and a further 2 allow control of stirring and purging operations in the user’s electrochemical cell. A high speed serial link allows data transfer between the XMOS Startkit and a host PC running LabVIEW, in the usual manner.
An image showing the PCB layout for the proposed new potentiostat is shown below. Check back here to see work in progress on this instrument.