9.1.6 Establishing Proper Limits on the Absolute Input Voltage

The ADS122C04 can be used to measure various types of input signal configurations: single-ended, pseudo-differential, and fully differential signals (which can be either unipolar or bipolar). However, configuring the device properly for the respective signal type is important.

Signals where the negative analog input is fixed and referenced to analog ground (V_AINN = 0 V) are commonly called single-ended signals. If the PGA is disabled and bypassed, the absolute input voltages of the ADS122C04 can be as low as 100 mV below AVSS and as large as 100 mV above AVDD. Therefore, the PGA_BYPASS bit must be set in order to measure single-ended signals when a unipolar analog supply is used (AVSS = 0 V). Gains of 1, 2, and 4 are still possible in this configuration. Measuring a 0-mA to 20-mA or 4-mA to 20-mA signal across a load resistor of 100 Ω referenced to GND is a typical example. The ADS122C04 can directly measure the signal across the load resistor using a unipolar supply, the internal 2.048-V reference, and gain = 1 when the PGA is bypassed.

If gains larger than 4 are needed to measure a single-ended signal, the PGA must be enabled. In this case, a bipolar supply is required for the ADS122C04 to meet the absolute input voltage requirement of the PGA.

Signals where the negative analog input (AIN_N) is fixed at a voltage other the 0 V are referred to as pseudo-differential signals.

Fully differential signals in contrast are defined as signals having a constant common-mode voltage where the positive and negative analog inputs swing 180° out-of-phase but have the same amplitude.

The ADS122C04 can measure pseudo-differential and fully differential signals with the PGA enabled or bypassed. However, the PGA must be enabled in order to use gains greater than 4. The absolute input voltages of the input signal must meet the absolute input voltage restrictions of the PGA (as explained in the PGA Input Voltage Requirements section) when the PGA is enabled. Setting the common-mode voltage at or near (AVSS + AVDD) / 2 in most cases satisfies the PGA absolute input voltage requirements.

Signals where both the positive and negative inputs are always ≥ 0 V are called unipolar signals. These signals can in general be measured with the ADS122C04 using a unipolar analog supply (AVSS = 0 V). As mentioned previously, the PGA must be bypassed in order to measure single-ended, unipolar signals when using a unipolar supply.

A signal is called bipolar when either the positive or negative input can swing below 0 V. A bipolar analog supply (such as AVDD = 2.5 V, AVSS = –2.5 V) is required in order to measure bipolar signals with the ADS122C04. A typical application task is measuring a single-ended, bipolar, ±10-V signal where AIN_N is fixed at 0 V and AIN_P swings between –10 V and 10 V. The ADS122C04 cannot directly measure this signal because the 10 V exceeds the analog power-supply limits. However, one possible solution is to use a bipolar analog supply (AVDD = 2.5 V, AVSS = –2.5 V), gain = 1, and a resistor divider in front of the ADS122C04. The resistor divider must divide the voltage down to ≤ ±2.048 V in order to measure the voltage using the internal 2.048-V reference.