1

¨

¨

©

§

1 -

1

2

2

-

+

580 k:

24.5 k:

LMV641

24.5 k:

580 k:

4.446V

4.554V

500:

500:

SENSOR

GAIN-BANDWIDTH PRODUCT

=

10 MHz

23.2

= 431 kHz

LMV641

SNOSAW3C – SEPTEMBER 2007 – REVISED FEBRUARY 2013

www.ti.com

Figure 47. Thevenin Equivalent Showing Required Gain

bridge is minimally loaded and the offset errors induced by the op amp stages are minimized. These resistors

should have 1% tolerance, or better, for the best noise rejection and offset minimization.

loop. It performs a temperature compensation function for the bridge so that it will have greater accuracy over a

wide range of operational temperatures. With mangetoresistive sensors, temperature drift of the bridge sensitivity

is negative and linear, and in the case of the sensor used here, is nominally

−3000 PP/M. Thus the gain of U2

needs to increase proportionally with increasing temperature, suggesting a thermistor with a positive temperature

the thermistor chosen, and is dependent on the thermistor’s %/°C shift in resistance. For best op amp

compatibility, the thermistor resistance should be greater than 1000

thermistor.

Gain Error and Bandwidth Consideration if Using an Analog to Digital Converter

The bandwidth available from Figure 43 is dependent on the system closed loop gain required and the maximum

gain-error allowed if driving an analog to digital converter (ADC). If the output from the sensor is intended to drive

an ADC, the bandwidth will be considerably reduced from the closed-loop corner frequency. This is because the

gain error of the pre-amplifier stage needs to be taken into account when calculating total error budget. Good

practice dictates that the gain error of the amplifier be less than or equal to half LSB (preferably less in order to

allow for other system errors that will eat up a portion of the available error budget) of the ADC. However, at the

−3 dB corner frequency the gain error for any amplifier is 29.3%. In reality, the gain starts rolling off long before

the

−3 dB corner is reached. For example, if the amplifier is driving an 8-bit ADC, the minimum gain error allowed

for half LSB would be approximately 0.2%. To achieve this gain error with the op amp, the maximum frequency

of interest can be no higher than

(4)

Given that the LMV641 has a GBW of 10 MHz, and is operating with a closed loop gain of 26.3, its closed loop

bandwidth is 380 kHZ, therefore

16

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