Datasheet

-
+
1/2
LMP2022
0.1 PF
-
+
1/2
LMP2022
0.1 PF
5.1 k:
0.1%
5.1 k:
0.1%
200:
200:
280:
V
A
V
A
+
-
V
A
LMP2021
ADC161S626
+
V
R
= 1/2 V
A
-
1 k:
1 k:
R
3
R
4
R
1
R
2
V
A
EMI
180:
470 pF
V
A
= 5V
LMP2021, LMP2022
www.ti.com SNOSAY9E –SEPTEMBER 2008–REVISED MARGH 2013
The circuit in Figure 50 shows a signal path solution for a typical bridge sensor using the LMP2021/LMP2022.
Bridge sensors are created by replacing at least one, and up to all four, of the resistors in a typical bridge with a
sensor whose resistance varies in response to an external stimulus. Using four sensors has the advantage of
increasing output dynamic range. Typical output voltage of one resistive pressure sensor is 2 mV per 1V of
bridge excitation voltage. Using four sensors, the output of the bridge is 8 mV per 1V. The bridge voltage is this
system is chosen to be 1/2 of the analog supply voltage and equal to the reference voltage of the ADC161S626,
2.5V. This excitation voltage results in 2.5V * 8 mV = 20 mV of differential output signal on the bridge. This 20
mV signal must be accurately amplified by the amplifier to best match the dynamic input range of the ADC. This
is done by using one LMP2022 and one LMP2021 in front of the ADC161S626. The gaining of this 20 mV signal
is achieved in 2 stages and through an instrumentation amplifier. The LMP2022 in Figure 50 amplifies each side
of the differential output of the bridge sensor by a gain 18. Bridge sensor measurements are usually done up to
10s of Hz. Placing a 300 Hz filter on the LMP2022 helps removing the higher frequency noise from this circuit.
This filter is created by placing two capacitors in the feedback path of the LMP2022 amplifiers. Using the
LMP2022 with a gain of 18 reduces the input referred voltage noise of the op amps and the system as a result.
Also, this gain allows direct filtering of the signal on the LMP2022 without compromising noise performance. The
differential output of the two amplifiers in the LMP2022 are then fed into a LMP2021 configured as a difference
amplifier. This stage has a gain of 5, with a total system having a gain of (18*2+1)*5 = 185. The LMP2021 has an
outstanding CMRR value of 139. This impressive CMRR improves system performance by removing the
common mode signal introduced by the bridge. With an overall gain of 185, the 20 mV differential input signal is
gained up to 3.7V. This utilizes the amplifiers output swing as well as the ADC's input dynamic range.
This amplified signal is then fed into the ADC161S626. The ADC161S626 is a 16-bit, 50 kSPS to 250 kSPS 5V
ADC. In order to utilize the maximum number of bits of the ADC161S626 in this configuration, a 2.5V reference
voltage is used. This 2.5V reference is also used to power the bridge sensor and the inverting input of the ADC.
Using the same voltage source for these three points helps reducing the total system error by eliminating error
due to source variations.
With this system, the output signal of the bridge sensor which can be up to 20 mV is accurately gained to the full
scale of the ADC and then digitized for further processing. The LMP2021/LMP2022 introduced minimal error to
the system and improved the signal quality by removing common model signals and high frequency noise.
Figure 50. LMP2021/LMP2022 used with ADC161S626
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