-

+

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:

VA

VA

+

-

VA

LMP2021

ADC161S626

+

VR = 1/2 VA

-

1 k:

1 k:

R3

R4

R1

R2

VA

EMI

180:

470 pF

VA = 5V

23

LMP2021, LMP2022

www.ti.com

SNOSAY9G – SEPTEMBER 2008 – REVISED FEBRUARY 2016

Product Folder Links: LMP2021 LMP2022

Submit Documentation Feedback

Copyright © 2008–2016, Texas Instruments Incorporated

8.2 Typical Application

Figure 46 shows the Bridge Sensor Interface for these devices.

Figure 46. LMP202x Used With ADC161S626

8.2.1 Design Requirements

Bridge sensors are used in a variety of applications such as pressure sensors and weigh scales. Bridge sensors

typically have a very small differential output signal. This very small differential signal needs to be accurately

amplified before it can be fed into an ADC. As discussed in the previous sections, the accuracy of the op amp

used as the ADC driver is essential to maintaining total system accuracy.

The high DC performance of the LMP202x make these amplifiers ideal choices for use with a bridge sensor. The

LMP202x have very low input offset voltage and very low input offset voltage drift. The open loop gain of the

LMP202x is 160 dB.

The circuit in Figure 46 shows a signal path solution for a typical bridge sensor using the LMP202x. Bridge

sensors are created by replacing at least one of the resistors in a typical bridge with a sensor whose resistance

varies in response to an external stimulus. For this example, the expected bridge output signal will be in the

range of ±12 mV. This 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 on chip EMI rejection filters available on the LMP202x help remove the EMI interference introduced to the

signal and hence improve the overall system performance.

8.2.2 Detailed Design Procedure

The amplification of this ±12 mV signal is achieved in 2 stages and through a three op-amp instrumentation

amplifier. The dual LMP2022 in Figure 46 amplifies each side of the differential output of the bridge sensor by a

gain of 18.2. Using the LMP2022 with a gain of 18.2 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 * 2 +1 )

* 5 = 187. 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 187, the

±12 mV differential input signal is gained up to ±2.24V (0.26 V to 4.74V single ended). This utilizes the amplifiers

output swing as well as the ADC's input dynamic range, and allows for some overload range.