Question

A push-pull capacitive displacement transducer is interfaced to a differential amplifier and an ADC as shown in the figure below.

Note that the bridge supply and the analog reference input for the ADC are derived from the same 10 V DC source.
$\left(\text{Gain of amplifier=40}\right)$

If the supply voltage to the bridge decreases by 5%, the sensitivity of the measurement system

• A. decreases by 5%
• B. does not change
• C. increases by 5%
• D. increases by 200%

Answer 1

The correct option is B does not change

If the supply voltage to the bridge is reduced by 5% then the reference voltage to A/D converter also reduced by 5%.

So,

$V_{in}=10V-0.05\times 10=9.5V$

$V_{ref}\text{to A/D converter = 9.5 V}$

Then at full scale displacement
$\frac{\Delta C}{C}=0.005$

Bridge output voltage,
$V_{Om}=\frac{V_{in}}{2}\cdot \frac{\Delta C}{C}$

Push - Pull arrangement,
$V_{OB}=\frac{9.5}{2}\times 0.05=0.2375$

Analog input to A/D converter,

$(V_{in}{)}_{ADC}=gainofamplifier\times V_{OB}$

$=40\times 0.2375=9.5V$

So, at full scale input displacement analog input to A/D converter is equals to analog reference input, then digital display will be full scale. This means there is no change in sensitivity.