The ADS131A0x analog inputs are directly connected to the switched-capacitor sampling network of the ΔΣ modulator without a multiplexer or integrated buffer. The device inputs are measured differentially (VIN = VAINxP – VAINxN) and can span from –VREF / Gain to VREF / Gain. Figure 38 shows a conceptual diagram of the modulator circuit charging and discharging the sampling capacitor through switches, although the actual implementation is slightly different. The timing for switches S1 and S2, as shown in Figure 37, are 180 degrees out-of-phase of one another.
Electrostatic discharge (ESD) circuitry protects the inputs. Figure 38 shows a simplified representation of the ESD circuit. Protection for input voltages exceeding AVDD can be modeled as a simple diode.
The negative charge pump voltage, VNCP, controls the voltage at which the low-side protection devices begin conducting. Tie VNCP to AVSS if the charge pump is not used to ensure the clamping voltage is properly set. The charge pump cannot provide a large amount of current. The mechanism shown in Figure 38 ensures current provided by the charge pump is limited in the case of an overvoltage event.
To prevent the ESD diodes from being enabled, the absolute voltage on any input must stay within the range provided by Equation 3 when the internal charge pump is disabled and within the range in Equation 4 when the internal charge pump is enabled:
If the voltages on the input pins have any potential to violate these conditions, external clamp diodes or series resistors may be required to limit the input currents to safe values (see the Absolute Maximum Ratings table).
The charging of the input capacitors draws a transient current from the sensor driving the ADS131A0x inputs. The average value of this current can be used to calculate an effective impedance of ZIN, where ZIN = VIN / IAVERAGE. This effective input impedance is a function of the modulator sampling frequency and Equation 5 can be used to calculate an estimate value. When using fMOD = 4.096 MHz, the input impedance is approximately 130 kΩ.
There are two general methods of driving the ADS131A0x analog inputs, as shown in Figure 39: pseudo-differential or fully-differential.
To apply a pseudo-differential signal to the fully-differential inputs, apply a dc voltage to AINxN, preferably to the analog mid-supply [(AVDD + AVSS) / 2] or [(AVDD + VNCP) / 2] when the negative charge pump is enabled. The AINxP pins can swing between –VREF / Gain to VREF / Gain (as shown in Figure 40) around the common voltage. The common-mode voltage, VCM, changes with VAINxP.
Configure the signals at AINxP and AINxN to be 180° out-of-phase centered around a common-mode voltage to use a fully-differential input method. Both the AINxP and AINxN inputs swing from VCM +½ VREF / Gain to VCM –½ VREF / Gain, as shown in Figure 41. The differential voltage at the maximum and minimum points is equal to VREF / Gain to –VREF / Gain, respectively. The VCM voltage remains fixed when AINxP and AINxN swing. Use the ADS131A0x in a differential configuration to maximize the dynamic range of the data converter. For optimal performance, the VCM is recommended to be set at the midpoint of the analog supplies.
Tie any unused analog input channels directly to AVSS.