t

SAMPLE

ON

OFF

S

1

S

2

OFF

ON

Equivalent

Circuit

f(MOD) = 250 kHz

ZCM

ZDIFF

ZCM

AINN

AINP

0.7 V

0.7 V

S1

S1

CA1

CB

CA2

S2

S2

0.7 V

0.7 V

AINN

AINP

12

ADS1018

SBAS526D – NOVEMBER 2012 – REVISED SEPTEMBER 2019

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Copyright © 2012–2019, Texas Instruments Incorporated

Feature Description (continued)

8.3.2 Analog Inputs

The ADS1018 uses a switched-capacitor input stage where capacitors are continuously charged and then

discharged to measure the voltage between AINP and AINN. This frequency at which the input signal is sampled

is called the sampling frequency or the modulator frequency (f(MOD)). The ADS1018 has a 1-MHz internal

oscillator which is further divided by a factor of 4 to generate the modulator frequency at 250 kHz. The capacitors

used in this input stage are small, and to external circuitry, the average loading appears resistive. This structure

is shown in Figure 9. The resistance is set by the capacitor values and the rate at which they are switched.

Figure 10 shows the setting of the switches illustrated in Figure 9. During the sampling phase, switches S1 are

closed. This event charges CA1 to V(AINP), CA2 to V(AINN), and CB to (V(AINP) – V(AINN)). During the discharge phase,

S1 is first opened and then S2 is closed. Both CA1 and CA2 then discharge to approximately 0.7 V and CB

discharges to 0 V. This charging draws a very small transient current from the source driving the ADS1018

analog inputs. The average value of this current can be used to calculate the effective impedance (Zeff), where

Zeff = VIN / IAVERAGE.

Figure 9. Simplified Analog Input Circuit

Figure 10. S1 and S2 Switch Timing

Common-mode input impedance is measured by applying a common-mode signal to the shorted AINP and AINN

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 M

Ω for the default full-scale range. In Figure 9, the

common-mode input impedance is ZCM.

The differential input impedance is measured by applying a differential signal to AINP and AINN 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 9, the differential input impedance is ZDIFF.

Make sure to consider the typical value of the input impedance. Unless the input source has a low impedance,

the ADS1018 input impedance may affect the measurement accuracy. For sources with high output impedance,

buffering may 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.