A differential analog input channel consists of two inputs, high side (positive) and low side (negative). The output measured is the difference between the two inputs, thus rejecting signals that are common to both inputs. In many situations this offers additional noise immunity to the signal of interest. The image below illustrates how to interface a sensor to a differential input channel. 

J8 jumper must be removed for differential mode of operation. This also configures the STX104 card for eight analog input channels. 

All analog inputs require a small amount of bias current, on the order of tens of nano-amps. If the sensor or signal source is not referenced to the STX104 AGND or system power supply then a resistor must be tied from one of the differential leads to AGND. The resistor value range is typically 100K ohms down to 100 ohms, depending on the application and the actual input bias current required. In most cases a 10K ohm resistor will do and will add less than +/-50uV of common-mode due to the bias current. Actually, the R_bias value could be as low as zero ohms, but using zero ohms you are reverting to single-ended behavior. 

Source resistance Rsource along with cable and input parasitic capacitances must be taken into considered to determine minimum settling time. More details can be found here. 

The input ranges for each half of the differential input along with the full differential input range.

NOTE: Analog Configuration Register (ACFG) 

To measure the output of a bridge network as shown in the diagram below. When the bridge is balanced, the common mode voltage at both the positive and negative inputs of the differential input are the same. Thus, the STX104 will report zero volts output (0x8000 in straight binary). Note that the voltage driving the bridge will be ratiometric at the outputs and thus nulled at the output. 

If the negative side of the source driving the bridge is tied to the system supply that supplies power to the STX104, then the R_bias is not required because input bias currents have a complete DC path. If you are not sure whether your signal source is referenced to the STX104 AGND, power down the system, and using an ohmmeter measure the resistance between STX104 AGND and the common side of your signal source, you will want the resistance to be less than 10K ohms. 

• Common mode rejection
• Signals can be extracted even though there is a common mode voltage
• typically improves noise immunity
• Can perform ratiometric measurements

Disadvantages

• More wiring
• Fewer channels available

Connectivity