The ADS111x-Q1 use a switched-capacitor input stage where capacitors are continuously charged and then discharged to measure the voltage between AIN_P and AIN_N. The frequency at which the input signal is sampled is called the sampling frequency or the modulator frequency (f_MOD). The ADS111x-Q1 has a 1-MHz internal oscillator that is further divided by a factor of 4 to generate f_MOD at 250 kHz. The capacitors used in this input stage are small, and to external circuitry, the average loading appears resistive. Figure 8-5 shows this structure. The capacitor values set the resistance and switching rate. Figure 8-6 shows the timing for the switches in Figure 8-5. During the sampling phase, switches S₁ are closed. This event charges C_A1 to V_(AINP), C_A2 to V_(AINN), and C_B to (V_(AINP) – V_(AINN)). During the discharge phase, S₁ is first opened and then S₂ is closed. Both C_A1 and C_A2 then discharge to approximately 0.7 V and C_B discharges to 0 V. This charging draws a very small transient current from the source driving the ADS111x-Q1 analog inputs. The average value of this current can be used to calculate the effective impedance (Z_eff), where Z_eff = V_IN / I_AVERAGE.

Figure 8-5 Simplified Analog Input Circuit

Figure 8-6 S₁ and S₂ Switch Timing

The common-mode input impedance is measured by applying a common-mode signal to the shorted AIN_P and AIN_N inputs and measuring the average current consumed by each pin. The common-mode input impedance changes depending on the full-scale range, but is approximately 6 MΩ for the default full-scale range. In Figure 8-5, the common-mode input impedance is Z_CM.

The differential input impedance is measured by applying a differential signal to AIN_P and AIN_N inputs where one input is held at 0.7 V. The current that flows through the pin connected to 0.7 V is the differential current and scales with the full-scale range. In Figure 8-5, the differential input impedance is Z_DIFF.

Make sure to consider the typical value of the input impedance. Unless the input source has a low impedance, the ADS111x-Q1 input impedance can affect the measurement accuracy. For sources with high-output impedance, buffering can be necessary. Active buffers introduce noise, and also introduce offset and gain errors. Consider all of these factors in high-accuracy applications.

The clock oscillator frequency drifts slightly with temperature; therefore, the input impedances also drift. For most applications, this input impedance drift is negligible, and can be ignored.