Signal conditioning The LT6370’s exceedingly low gain
error (<0.084 per cent at G = 10 V/V) and low input offset voltage (<50 µV max specified over temperature) guarantee that U2 is presented with a true replica of the sensor voltage, minus the interference picked up by the UTP, to compare against the reference voltage developed at U2’s inver ting input. The LT6657-5 creates a stable, low noise, and low drift voltage reference, immunising the entire circuit from supply voltage variation. Of par ticular impor tance is the LT6657-5’s low 1/f noise, which can have a significant contribution due to the large gain in the circuit. With the simple RC low-pass filters
Figure 5: A single UTP for remote sensing
For the component values shown in Figure 5, there will be about 1 mA flowing through the
sensor RSENSOR. With U1’s RG1 value, that stage runs at G = 10 V/V and provides a 10× replica of
the voltage across RSENSOR at its output voltage, about 3.5 V. U1’s main task is to eliminate the interference present on the UTP long length of wire and responding only to the sensor voltage which is the sensor resistance times the ~1 mA current flowing through it. LT6370’s low offset voltage and drift along with its exceptional CMR make it the obvious choice. The other half of the Wheatstone bridge is
comprised of R5, R6, and VR1 with near identical current flow as the sensor half of the bridge. Both the sensor voltage at U1 output and the reference voltage at VR1 wiper reach the differential inputs of U2 after some low-pass filtering to eliminate unwanted noise. U2 is set for high gain (G = 1 + 24.2 kΩ/RG2 = 100 V/V) to magnify the very small sensor voltage on its positive input compared against the fixed, low noise reference voltage, derived from the LT6657-5 voltage reference, on its negative input. U1 output accurately represents the measured strain applied to the sensor, attached to the element or material of interest, to drive an ADC or other similar signal processing. The optional DAC and OPA (U4, U5) tied to
U2’s REF pin (which can be grounded if no offset adjust is needed) can be used to provide output offset adjustment and zeroing. By using the DAC, it is possible to shift the U2 output voltage to a desired pedestal or CM level suitable for the selected ADC. For example, an ADC with a reference voltage of 5 V can be driven directly from U2 with its zero output set to 2.5 V using the DAC driving U2 REF input. Done this way, 0 V to 2.5 V ADC analogue input represents compression and 2.5 V to 5 V signal represents tension strain. It is important to note that the device driving U2 REF pin, AD820 in this case, should maintain a low impedance to eliminate any possible gain errors. Here is the expression for the output voltage as a function of the sensor resistance and relationship
62
where ΔRSENSOR is the change in sensor resistance due to strain
between the output voltage and the strain (ε) being measured:
(R9, C2 and R10, C3) set to roll-off at about 10 Hz on each input of U2, the output noise can be reduced by limiting the bandwidth. The low (<10 Hz) LT6370 1/f noise corner frequency, as shown in Figure 6, provides an advantage by reducing the impact of the 1/f noise. Fur thermore, the current noise density plot shows that it is much better to keep both input impedances balanced for the lowest current noise impact by taking advantage of the correlated component of noise at the input(s). Hence the value for R10 is reduced to 3.74 kΩ in order to match the R9 impedance of 4.75 kΩ due to the equivalent impedance looking into the wiper of VR1. Placing a bridge sensor at a distance
Where:
L refers to the sensor length ε refers to the amount of strain being measured For the sensor chosen:
Rsensor = 350 Ω GF= 2 Resulting in the strain (ε):
from a signal processing amplifier requires an instrumentation amplifier that can cleanly extract the measured differential voltage. The attributes of the LT6370 instrumentation amplifier enable it to process successfully signals from distant sensors via long cable runs. The LT6370 manufacturing process, which invokes on- chip heaters during production testing to guarantee over temperature drift values, fur ther enhances the LT6370 suitability to remote monitoring applications, and improves longevity and product life in hard-to-service installations.
Analog Devices
www.analog.com
Figure 6: LT6370 input referred current/voltage noise density March 2019 Instrumentation Monthly
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