AN135 Datasheet, PDF (1/5 Pages) Xicor Inc. – Sensor Circuits and Digitally Controlled Potentiometers

Application Note
AN135
Sensor Circuits and Digitally Controlled Potentiometers
Steve Woodward, University of North Carolina
Chuck Wojslaw, Xicor, Inc.
OBJECTIVE
The objective of this application note is to (1) illustrate
the use of Xicor’s digitally controlled potentiometers
(XDCPs) to automate the control of the key parameters
of sensor circuits and (2) provide the design engineer
with reference designs.
DESCRIPTION
Sensors are energy conversion devices. Sensors or
transducers convert the physical world of light, pressure,
temperature, flow, level, acceleration, force, etc. to
electrical signals but generally they can’t do so without
help. Excitation and/or signal conditioning electronics
are almost always needed to interface the sensor and
provide adjustable calibration, amplification, linear-
ization, and level transformation functions.
Digitally controlled potentiometers add variability and
programmability to the sensor circuit and provide an
automated alternative to manually adjusted mechanical
trimmers. The results are accuracy, speed, reliability,
packaging flexibility, and labor and cost savings.
The following three circuits illustrate the use of the
digitally controlled potentiometer in sensor circuits. In a
photodiode’s transimpedance amplifier, one XDCP
provides for the adjustment of a wide range current-to-
voltage conversion function while a second XDCP
provides a precision zero trim. Two digitally controlled
potentiometers provide for the zero adjust and full scale
adjust features of a presssure transducer signal
conditioning circuit. Similarly, two potentiometers
provide the zero adjust and full scale adjust features of
a thermometer circuit using a platinum resistance
temperature detector. These circuits illustrate basic
ideas in the design of sensor circuits and are but a small
sampling of the many potential applications in this area.
PHOTOVOLTAIC TRANSIMPEDANCE AMPLIFIER
Photovoltaic detectors are used to sense and measure
light energy in industrial, medical, consumer, and
scientific instrumentation applications. Solid state
photovoltaics respond to wavelengths ranging from the
far infra-red through the visible spectrum and into the
ultra-violet and therefore tend to excel in precision
photometric applications. This versatility suits
photovoltaics to such diverse jobs as chemical spectral
analysis, colorimetry, non-contact thermometry, flame
detection, and non-invasive blood-gas monitoring.
The basic signal conditioning circuit for photovoltaics is
the current-to-voltage or transimpedance amplifier of
Figure 1. The chief shortcoming of this classic circuit is
the inability of one value of feedback resistor to
adequately accommodate the four, five, or more
decades of dynamic range of current produced by many
of the photo detectors. Even when R_F is made
adjustable, the finite resolution of the feedback
resistance fails to fully resolve the dynamic range
problem.
RF
IS
–
VO = –IS RF
+
Figure 1. Transimpedance Amplifier
The circuit of Figure 2 combines two Xicor X9258T
digitally controlled potentiometers with an AD822 low-
noise dual opamp to create a flexible, digitally
calibrated, wide dynamic range transimpedance
amplifier topology that can be used with virtually any
photovoltaic detector technology. The amplifier output is
given by:
V_O = I_S(1MΩ)_{2----15---6-+----–}-P---P--1---1--
where P1 is the 8-bit (0 to 255) digital value written to
DCP1.
AN135-1