ADS1118_13 Datasheet, PDF (15/38 Pages) Texas Instruments – Ultra-Small, Low-Power, SPI™-Compatible, 16-Bit Analog-to-Digital Converter and Temperature Sensor with Internal Reference

ADS1118
www.ti.com
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
ANALOG INPUTS
The ADS1118 uses a switched-capacitor input stage where capacitors are continuously charged and then
discharged to measure the voltage between AIN_P and AIN_N. The capacitors used are small, and to external
circuitry, the average loading appears resistive. This structure is shown in Figure 40. The resistance is set by the
capacitor values and the rate at which they are switched. Figure 41 shows the on/off setting of the switches
illustrated in Figure 40. During the sampling phase, switches S₁ are closed. This event charges C_A1 to AIN_P, C_A2
to AIN_N, and C_B to (AIN_P – AIN_N). 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 ADS1118 analog inputs. The average value of this current can be
used to calculate the effective impedance (R_eff), where R_eff = V_IN / I_AVERAGE.
0.7V
AIN_P
S₁
S₁
AIN_N
C_A1
S₂
C_B
S₂
C_A2
0.7V
Equivalent
Circuit
0.7V
f_CLK = 250kHz
AIN_P
AIN_N
Figure 40. Simplified Analog Input Circuit
Z_CM
Z_DIFF
Z_CM
0.7V
ON
S₁
OFF
t_SAMPLE
ON
S₂
OFF
Figure 41. S₁ and S₂ Switch Timing for Figure 40
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 PGA gain setting, but is approximately 6MΩ for the default PGA gain setting. In
Figure 40, 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 PGA gain setting. In Figure 40, the differential input impedance is Z_DIFF. Table 1 describes the
typical differential input impedance.
Table 1. Differential Input Impedance
FS (V)
±6.144^{(1) (1)}
±4.096^{(1) (1)}
±2.048
±1.024
±0.512
±0.256
DIFFERENTIAL INPUT IMPEDANCE
22 MΩ
15 MΩ
4.9 MΩ
2.4 MΩ
710 kΩ
710 kΩ
(1) This parameter expresses the full-scale range of the ADC scaling. In
no event should more than VDD + 0.3V be applied to this device.
Copyright © 2010–2013, Texas Instruments Incorporated
Product Folder Links: ADS1118
Submit Documentation Feedback
15