ADS1018_14 datasheet(11/37 Pages) TI1

Part #	ADS1018

Description	Ultra-Small, Low-Power, SPI??Compatible, 12-Bit, Analog-to-Digital Converter and Temperature Sensor with Internal Reference

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t

SAMPLE

ON

OFF

S

1

S

2

OFF

ON

Equivalent

Circuit

f

= 250 kHz

CLK

Z

CM

Z

DIFF

Z

CM

AIN

N

AIN

P

0.7 V

S

1

S

1

C

A1

C

B

C

A2

S

2

S

2

0.7 V

AIN

N

AIN

P

ADS1018

www.ti.com

SBAS526B – NOVEMBER 2012 – REVISED OCTOBER 2013

ANALOG INPUTS

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

discharged to measure the voltage between AINP and AINN. The capacitors used are small, and to external

circuitry, the average loading appears resistive. This structure is shown in Figure 11. The resistance is set by the

capacitor values and the rate at which they are switched. Figure 12 shows the on/off setting of the switches in

Figure 11. During the sampling phase, all S1 switches are closed. This event charges CA1 to AINP, CA2 to AINN,

and CB to (AINP – 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 (Reff), where Reff = VIN / IAVERAGE.

Figure 11. Simplified Analog Input Circuit

Figure 12. S1 and S2 Switch Timing for Figure 11

The 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 PGA gain setting, but is approximately 6 M

Ω for the default PGA gain setting. In

Figure 11, 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 PGA gain setting. In Figure 11, the differential input impedance is ZDIFF. Table 1 describes the

typical differential input impedance.

Table 1. Differential Input Impedance

FS (V)

DIFFERENTIAL INPUT IMPEDANCE

±6.144(1)

22 M

Ω

±4.096(1)

15 M

Ω

±2.048

4.9 M

Ω

±1.024

2.4 M

Ω

±0.512

710 k

Ω

±0.256

710 k

Ω

(1)

This parameter expresses the full-scale range of the ADC scaling. In

no event should more than VDD + 0.3 V be applied to this device.

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