-

+

VO = 2.50V

580 k:

24.5 k:

LMV641

24.5 k:

580 k:

4.446V

4.554V

500:

500:

SENSOR

BW-3 dB =

GAIN-BANDWIDTH PRODUCT

AVCL

=

10 MHz

23.2

= 431 kHz

AVCL =

R4

RTHEV + R2

= 23.2

21

LMV641

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Typical Applications (continued)

Referring to the simplified diagram in Figure 43, and assuming that required full scale at the output of the

amplifier is 2.5 V, a gain of 23.2 is needed for U1. It is clear from the Thevenin equivalent circuit in Figure 45 that

a sensor Thevenin equivalent source resistance, RTHEV, of 500 Ω will be in series with both the inverting and

noninverting inputs of the LMV641. Therefore, the required gain is:

(4)

Choosing R1 = R2 = 24.5 kΩ, then R4 will be approximately 580 kΩ. The actual values chosen will depend on the

full-scale needs of the succeeding circuitry as well as bandwidth requirements. The values shown here provide a

−3-dB bandwidth of approximately 431 kHz, and are found as follows.

Figure 45. Thevenin Equivalent Showing Required Gain

By choosing input resistor values for R1 and R2 that are four to ten times the bridge element resistance, the

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.

Referring once again to Figure 41, U2 is an additional gain stage with a thermistor element, RTH, in the feedback

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

coefficient. Selection of the temperature compensation resistor, RTH, depends on the additional gain required, on

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

Ω. RTH should also be much less than RA,

the feedback resistor. Because the temperature coefficient of the AMR bridge is largely linear, RTH also needs to

behave in a linear fashion with temperature, thus RA is placed in parallel with RTH, which acts to linearize the

thermistor.

8.2.2.2.1

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

The bandwidth available from Figure 41 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