10.1.4 Establishing a Proper Input Voltage

The ADS114S0x 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. The input voltage of a single-ended signal consequently varies between 0 V and V_IN. If the PGA is disabled and bypassed, the input voltage of the ADS114S08 can be as low as 50 mV below AVSS and as large as 50 mV above AVDD. Therefore, set the PGA_EN bits to 10 in the gain setting register (03h) to measure single-ended signals when a unipolar analog supply is used (AVSS = 0 V). Only a gain of 1 is 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 ADS114S0x can directly measure the signal across the load resistor using a unipolar supply, the internal 2.5-V reference, and gain = 1 when the PGA is bypassed.

If gain is needed to measure a single-ended signal, the PGA must be enabled. In this case, a bipolar supply is required for the ADS114S0x to meet the 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. The input voltage of a pseudo-differential signal varies between V_AINN and V_AINN + V_IN.

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 ADS114S0x can measure pseudo-differential and fully-differential signals both with the PGA enabled or bypassed. However, the PGA must be enabled in order to measure any input with a gain greater than 1. The input voltage must meet the input and output voltage restrictions of the PGA, as explained in the PGA Input-Voltage Requirements section when the PGA is enabled. Setting the input voltage at or near (AVSS + AVDD) / 2 in most cases satisfies the PGA 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 ADS114S0x 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 ADS114S0x. 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 ADS114S0x cannot directly measure this signal because the 10-V signal 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 ADS114S0x. The resistor divider must divide the voltage down to ≤ ±2.5 V to be able to measure the voltage using the internal 2.5-V reference.