Page 301 - Mechatronics with Experiments

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JWST499-c05
JWST499-Cetinkunt
ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 287
Single-ended signal: V = V – V = V – V GND = V 2
1
2
in
2
Shield Twisted pair
V V
2 in
+ +
V 1
– –
R
V = ( )
1 +
f
R i out V in
GND R i
R f
(a)

Differential-ended signal: V = (V – V GND ) – (V – V GND ) = V – V 1
2
2
in
1
Shield Twisted pair
V 2 R
+ +
V
– 1 –
V out = K (V 2 – V 1 )
R
GND
KR
KR

(b)
FIGURE 5.26: (a) Single-ended signal, (b) differential-ended signal and operational amplifier.

a 16-channel ADC (plus a ground wire for zero reference voltage, hence a total of 17
wires) may be configured to handle 16 single-ended (SE) signals or 8 differential-ended
(DE) signals (Figure 5.27a and b). Both single-ended and differential-ended signals should
always be grounded only at one end (source or destination), not at both ends, in order to
avoid ground loop related noise.
In Figure 5.26a, a single-ended signal is connected to an operational amplifier. If
there is external noise, it will be additive to the signal line (V ) that is not grounded,
2
while the grounded line (V ) will dump the noise quickly. As a result, external noise will
1
be amplified with the same gain as the desired signal component. Figure 5.26b shows a
differential-ended signal connected to a differential op-amp. Differential amplifier inputs
are not grounded. It amplifies the difference between the two inputs. None of the inputs are
grounded, rather each carries a voltage level relative to the ground (GND). The actual signal
information is the difference between the two lines. When unwanted noise is induced, it is
fairly accurate to assume that it will be additive to both of the lines by the same amount.
Hence, the difference taken at the amplifier will cancel out this so called common-mode
signal. As long as the noise signal is induced equally on both conductors, to differential
amplifier can effectively cancel the noise.