DrivingStrain

The MAX1452's internal 75kΩ resistors can serve for R_ISRCand R_STC, or external resistors can be routed by switches SW1 and SW2, as shown in Figure 5. The ISRC pin gives access to the op amp and allows voltage feedback from the bridge drive. Figures 6, 7, and 8 depict three different voltage-drive circuits.


Figure 6. Circuit diagram for high-resistance sensors, using no external devices.


Figure 7. Circuit diagram featuring npn transistor for low-resistance sensors.


Figure 8. Circuit using external R_SUPPdrive.

For high-resistance sensors of 2kΩ and above, the simple circuit of Figure 6 provides voltage-drive excitation to the bridge. Opening SW1 and SW2 disables the FSOTC DAC modulation circuit. The op-amp feedback loop is completed by connecting pin ISRC to BDR, thereby obtaining feedback from the bridge excitation voltage. By sourcing current to the bridge, transistors T1 and T2 (in parallel) raise the bridge voltage to equal the FSO DAC voltage.

Low resistance (120Ω to 2kΩ) strain gauges or thick-film resistors wired in a Wheatstone bridge circuit cannot be driven from T2 directly. Use of an external npn transistor in an emitter-follower configuration (Figure 7) solves this problem. Current through the npn transistor is drawn directly from the VDD power rail at the collector. Op amp U1 raises the bridge voltage by driving T1 and T2 sufficiently into conduction to turn on the npn transistor. To close the loop, the bridge voltage at ISRC is fed back to the op amp. Bridge voltage is regulated to match the FSO DAC output voltage, and a small 0.1µF capacitor is added across the bridge for stability.

The npn transistor's base-emitter voltage (V_BE) has a significant temperature coefficient, but that effect is regulated out of the equation by feedback to U1. At cold temperature, where V_BEis large, the maximum bridge voltage is limited:

VBRIDGE_MAX= VDD - VT2_SAT- V_BE

Like the V_BEtempco, the gain of TNPN has a temperature component that is regulated out of the equation by the control feedback loop.

Another method of supplying sufficient drive current to a low-resistance bridge is to add a small external resistor in parallel with T2 (R_SUPPin Figure 8). The R_SUPPvalue ensures that the bridge voltage is slightly less than the desired value (3.0V for VDD = 5.0V). T2 then supplies additional current that raises the bridge voltage to the desired value. Because T2 in the OFF state is the lowest current T2 can supply, R_SUPPshould be sized for the worst-case low bridge voltage. Also, T2's maximum current capability (2mA at VBDR = 4.0V) determines the maximum bridge-voltage modulation allowable. This circuit is useful for bridge sensors with a relatively low temperature coefficient of sensitivity (TCS) which does not require significant bridge-voltage modulation.

Sensitivity effects introduced by the temperature coefficient of R_SUPPare regulated out by the feedback to U1. To assure an adequate drive-current margin when designing the circuit, be sure to consider the power-derated maximum and minimum for R_SUPP.

The MAX1452's flexible approach to bridge excitation gives the user a substantial amount of design freedom. This article has focused on voltage drive with and without a current boost, but many other bridge-drive configurations can be implemented. Other design considerations include the use of external temperature sensors on the control loop, and achieving sensor linearization (i。, nonlinearity with re大田县中钢网spect to th大田大田县欧美婬乱私人影院县老爸快跑e meas大田县拆迁法ured p大田县夸老公那方面厉害的句子arameter) by feeding the OUT signal into this loop.

MAX1455 pdf datasheet