Signal Management 5
5.9 Input and Source Impedance
As illustrated below, the input impedance (Ri) of an
VADC = VT x Ri / (Rs + Ri)
The input impedance (Ri) of most ADCs is at least 1 MΩ; low source impedance (Rs) usually presents no problem. Some transducers, such as piezoelectric types, have high source impedance, and should therefore be used with a charge- sensitive amplifier of low output impedance. As described in the following paragraphs, multiplexing can greatly reduce the effective input impedance of an
Figure 5-5. Analog to Digital Converter
5.10 Crosstalk
Crosstalk is a type of noise related to source impedance and capacitance, in which signals from one channel leak into an adjacent channel, resulting in interference or signal distortion. The impact of source impedance and stray capacitance can be estimated by using the following equation.
T = RC
Where T is the time constant, R is the source impedance, and C is the stray capacitance.
High source (transducer) impedance can be a problem in multiplexed A/D systems. When using more than 1 channel, the channel input signals are multiplexed into the A/D. The multiplexer samples each signal and then switches to the next input signal. A
A solution to high source impedance in relation to multiplexers involves the use of buffers. The term buffer has several meanings; but in this case, buffer refers to an operational amplifier having high input impedance but very low output impedance. Placing such a buffer on each channel (between the transducer and the multiplexer) prevents the multiplexer’s stray capacitance from combining with the high input impedance. This use of a buffer also stops transient signals from propagating backwards from the multiplexer to the transducer.