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Bridge Measurements Using
In-System Programmable
Analog Circuits
Introduction
This application note outlines the use of the ispPAC™10
in differential circuits for bridge networks. The ispPAC10
architecture is well suited for instrumentation measure-
ment since both inputs and outputs are differential.
In-System Programmability (ISP™) and programmable
gain control allow the user to reconfigure the device
characteristics after it is soldered onto a circuit board.
System offset errors can be calibrated out and gain errors
can be adjusted using the ispPAC10 as a programmable
gain stage.
Figure 2. Typical Passive Bridge
V
S
5V
R1
B
V2
R2
V1
A
R3
R4
R+∆R
GND
ispPAC10 Overview
The ispPAC10 contains four programmable analog mod-
ules called PACblocks. Refer to Figure 1 for the basic
structure of the PACblock. Each PACblock contains a
differential-output summing amplifier (OA) and two differ-
ential-input instrumentation amplifiers (IAs) with variable
gains of
±1
to
±10
in integer steps. The OA’s feedback
path contains a resistive element which can be switched
in or out, as well as a programmable capacitor array that
allows for more than 120 poles when the ispPAC10
device is used as an active filter. Thus, each PACblock
has the ability to sum two differential signals with inde-
pendently-selectable gain and inversion settings and to
act as a gain element (with the feedback switch closed)
or as an integrator (with the feedback switch open).
The gain settings, feedback, capacitor values and inter-
nal interconnects between PACblocks are configurable
through non-volatile E
2
CMOS
®
cells internal to the
ispPAC10. The device configuration is set by software
and downloaded to the device via a JTAG download
cable.
Figure 1. A Single PACblock
-10 to 10
PACblock 1
IA1
IN1
IA2
-10 to 10
OA1
2.5V
Bridge Circuits
Bridge circuits fundamentally contain a group of ele-
ments in which at least one element’s characteristics
change with applied external stimulus. The stimuli may
include temperature, force, pressure or strain. Figure 2
shows a typical bridge network formed from passive
elements.
The main function of the bridge is to expose small
changes in voltage or current caused by a perturbation of
the bridge structure. The small voltage deviations can
range from a few microvolts to hundreds of millivolts.
Without the bridge’s balancing function, it is very difficult
to resolve these deviations relative to the entire voltage
that appears across an individual element.
The small voltage change that is the output of most
bridges can be amplified using the ispPAC10. The gain of
each PACblock can be as high as
±10.
Using the four
PACblocks in an ispPAC10, a gain of up to 10,000 (80 dB)
can be realized. The device’s feedback capacitors can
also be chosen for lowest noise, if desired.
When the branch elements are arranged in the standard
four-element bridge configuration, the desired output is
the difference in node voltages between node ‘A’ and
node ‘B.’ Since the ispPAC10 inputs are differential,
circuit connections are easily made while the component
count is kept to a minimum. In order to interface properly
to the bridge, high impedance differential inputs are
OUT1
needed. The input impedance of the ispPAC10 is typi-
cally 1x10
9
ohms, more than high enough for the
measurement accuracy to not be affected due to input
loading. The outputs of the ispPAC10 are also differen-
an6006_01
1
September 1999
Bridge Measurements Using
In-System Programmable Analog Circuits
tial, making its integration into other measurement cir-
cuitry easier. And, the input and output offsets of the
ispPAC10 are typically 200
µV
after the device’s auto-
matic input offset calibration, making it one of the
lower-offset devices available.
Another advantage of the ispPAC10 is in-system-pro-
grammability, which makes it possible to reconfigure the
device for various load sensitivities or sensor types after
it is installed. This helps to make the ispPAC10 a versatile
solution for industrial-grade differential measurements.
There are many subtle issues that arise when trying to
measure small signals in the presence of noise and
distortion-causing effects. Included are thermal issues
and thermocouple effects due to dissimilar metals used
in the contacts of the sensor. It is also important to
understand the linearity of the sensor and how it affects
the interface to the measurement circuit.
Most resistive bridge circuits are based upon the fact that
the sensor element changes resistance with an applied
external stimulus such as force or pressure. For a single
resistive strain-gage element, the equivalent mathemati-
cal function is expressed as R+(∆R), where the
∆R
is the
value that changes with applied external stimulus. A
problem arises when there is also a change in tempera-
ture, because the sensor will now be affected by two
stimuli. If the effects of the temperature coefficient are
limiting the overall resolution and accuracy of the mea-
surement, then the bridge can be set up with temperature
compensation included.
There are simple methods to compensate for changes in
the sensor resistance due to temperature. The easiest
solution is to place multiple sensors in the bridge. For
resistive-element bridge setups, the main sensor ele-
ment can have a complement element mounted on the
same thermal substrate. The effects of temperature
changes are cancelled out due to the fact that the dummy
gage has the same temperature coefficient as the sensor
gage. See Figure 3 for this configuration.
Equations for a single sensor:
V
OUT
=
(
V
1
−
V
2
)
R
R
+ ∆
R
V
OUT
=
(
V
S
)
*
−
2
R
+ ∆
R
2
R
∆
R
V
OUT
=
(
V
S
)
*
4
R
+
2
∆
R
 ∆
R
V
OUT
≅
(
V
S
)
*
4
R
Figure 3. Bridge with Temperature Comp. Gage
V
S
5V
R1
B
V2
R2
V1
A
R3
Temperature
Comp. Gage
R4
R+∆R
Working Gage
GND
The approximation for V
OUT
assumes that the denomina-
tor for this function is approximately 4R due to the fact
that the small
∆R
is much smaller than R and can
therefore be neglected.
Most manufacturers design strain gage elements with
complementary branches for temperature compensa-
tion. These are typically arranged in an orthogonal layout
such that one gage is parallel to the strain direction and
the dummy gage is rotated 90 degrees so it is not affected
by the strain and adds only temperature compensation.
See Figure 4 for this compensation gage.
Figure 4. Strain Gage with Temp. Comp. Element
V2
V1
R3
Force
R4
Force
Temperature
Comp. Gage
Working Gage
To increase the gain of the bridge network itself, a second
element can be added in the opposite branch of the
bridge along with its complement as a temperature com-
pensator. This is shown in Figure 5. This configuration
yields the following equations:
2
Bridge Measurements Using
In-System Programmable Analog Circuits
Equations for two sensors:
V
OUT
=
V
1
−
V
2
Figure 6. Four-element Bridge
V
S
R+∆R
R1
B
V2
5V
R-∆R
R2
V1
A
R
R
+ ∆
R
−
V
OUT
=
(
V
S
)
2
R
+ ∆
R
2
R
+ ∆
R
 ∆
R
V
OUT
=
(
V
S
)
*
2
R
+ ∆
R
 ∆
R
V
OUT
≅
(
V
S
)
*
2
R
R3
R-∆R
GND
R4
R+∆R
Figure 5. Two-element Bridge with Temp. Comp.
V
S
5V
Working Gage
R+∆R
R1
B
V2
Temperature
Comp. Gage
R2
V1
A
Please refer to specific gage manufacturers for specifica-
tions on mounting techniques, linearity and temperature
coefficients. Web sites such as sensorsmag.com offer
large lists of sensor manufacturers.
The ispPAC10 will easily interface to any type of bridge
network, from a single element coupled with other resis-
tors to a full bridge with all four elements acted on by the
stimulus. The inputs to the PAC10 are differential and
must be within the bounds for its input common mode
voltage, centered around 2.5V. Therefore, it is easier to
use a bridge that uses a 5V supply, making the V1 and V2
nodes centered at 2.5V due to the voltage divider action
of the bridge.
Figure 7 shows a typical configuration for a two-element
differential bridge measurement. This configuration can
be used for various types of bridges and can be
reconfigured for gain changes due to bridge element
limitations. The gain shown above is 100 overall, with
each PAC10 block programmed for a gain of 10 (recall
that each PACblock is capable of independent gain
programming in integer steps of
±1
to
±10).
The differen-
tial output can then be sampled by a differential A/D
converter and converted to a digital value for further
processing.
Another application of the ispPAC10 for bridge measure-
ments is to drive the bridge using the differential outputs
of the device. This allows referencing the excitation
voltage to the ispPAC10’s stable VREF
OUT
, as well as
isolating the bridge from a noisy supply or ground. The
ispPAC10 device can be set up to supply a differential 3V
excitation, and its 10mA minimum guaranteed output
current is more than adequate for many bridge devices.
Further details can be found in AN6005,
ispPAC10:
Complete Interface for Bridge Sensor to 12-bit ADC.
R3
R4
Temperature
Comp. Gage
GND
R+∆R
Working Gage
Further gain and linearity improvements can be achieved
by placing the elements on opposite sides of the stressed
member. With this configuration, the linearity and the
gain are enhanced because one set of gages is under
compression while the other set is under tension. The
overall output is now more linear with stress and compen-
sated for temperature (Figure 6). The four-gage bridge
gives the most accurate measurements because of these
advantages. The equations for a four-element bridge are
shown below:
Equations for four sensors:
V
OUT
=
V
1
−
V
2
R
+ ∆
R R
− ∆
R
V
OUT
=
(
V
S
)
*
−
2
R
2
R
 ∆
R
V
OUT
≅
(
V
S
)
*
R
3
Bridge Measurements Using
In-System Programmable Analog Circuits
Figure 7. isPAC10 Configured as a Differential Bridge Gain Stage
V
S
R+∆R
R1
B
V2
5V
R2
V1 A
10
IA1
PACblock 1
61.6 pF
Output with gain of 10
OA1
2.5V
OUT1
R3
GND
R4
R+∆R
IN1
IA2
1
10
IA3
PACblock 2
61.6 pF
Output with gain of 100
OA2
2.5V
OUT2
IA4
1
Small Step Gain Adjustments
For calibration of the gain and small step adjustments, an
external voltage divider can be used to increase the
resolution of the gain steps to as high as 0.1% This topic
is outlined in more detail in Application Note AN6007,
In-
System Programmable Gain with Fractional Gain
Adjustments.
Please refer to AN6007 and the ispPAC10
Data Sheet for further details.
Support Assistance
Toll Free Hotline:
International:
FAX:
E-mail:
Internet:
1-888-477-7537
1-503-268-8000
1-503-268-8037
ispPACs@latticesemi.com
http://www.latticesemi.com
Summary
The ispPAC10 easily interfaces to any type of bridge
network, from single element circuits with temperature
compensation to four-element circuits where all four
elements are acted on by the stimulus. In particular, the
very-high-impedance differential inputs and the low-off-
set-voltage balanced outputs of the ispPAC10 make it
ideal for use in bridge measurement circuits. The device
also offers the advantages of a flexible architecture and
the capability to reconfigure the device parameters, such
as gain and filter characteristics, while the device is on a
circuit board. Gain factors can be set or changed by
software and the part can be reconfigured by external
downloading or by an embedded microprocessor, local
to the board or system.
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