One of the most common questions asked is the difference between single-ended and differential
inputs, and what applications they should be considered in.
First, a simple definition:
- DIFFERENTIAL INPUTS - A signal input circuit where SIGNAL LO and SIGNAL HI are electrically
floating with respect to ANALOG GROUND. For example, a differential input A/D card will have one HI (+)
and one LOW (-) pin for each input. There will also be a LLGND (LOW LEVEL GROUND) pin which may be
used if a ground connection is required. This allows the measurement of the voltage difference
between two signals tied to the same ground and provides superior common-mode noise rejection.
Where should differential inputs be used? Whenever electromagnetic interference (EMI) or radio frequency
interference (RFI) is present, a voltage can be induced on BOTH signal wires. A differential input
amplifier will reject the COMMON MODE VOLTAGE, provided that the common mode voltage plus the input
signal does not exceed the device's CMR specification. The effect on a single-ended input is usually
a voltage fluctuation between signal high and signal ground.
- SINGLE-ENDED INPUTS - A single-ended input has no commond mode range because there is only ONE
low wire, which is shared by all inputs. For example, if you have an A/D board with 16 single-ended inputs,
there will be 16 HIGH (+) lines and one LOW (-) line (sometimes called LLGND). Some cards may have several
LOW lines to provide extra places to make your ground connection, however, these lines are tied together
and are basically the same thing.
WHEN TO USE SINGLE-ENDED OR DIFFERENTIAL INPUTS
Differential inputs provide a more stable reading when EMI or RFI is present, and therefore, it is
recommended to use them whenever noise is generally a problem. This is especially true when measuring
THERMOCOUPLE, STRAIN GAGE and BRIDGE TYPE PRESSURE SENSOR inputs, since they produce very small signals
that are very succeptible to noise.
Single-ended inputs are lower in cost, and provide twice the number of inputs for the same size wiring
connector, since they require only one analog HIGH (+) input per channel and one LLGND (-) shared by all
inputs. Differential inputs require signal HIGH and LOW inputs for each channel and one common shared LLGND.
Single-ended inputs save connector space, cost, and are easier to install.