-
+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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SNOSAW3D –SEPTEMBER 2007–REVISED AUGUST 2016
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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 Ωwill 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 R4will 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 R1and R2that 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 RAis 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