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

ADS1118
SBAS457C – OCTOBER 2010 – REVISED FEBRUARY 2013
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The typical value of the input impedance cannot be neglected. Unless the input source has a low impedance, the
ADS1118 input impedance may affect the measurement accuracy. For sources with high output impedance,
buffering may be necessary. Note that active buffers introduce noise, and also introduce offset and gain errors.
All of these factors should be considered in high-accuracy applications.
Because the clock oscillator frequency drifts slightly with temperature, the input impedances also drift. For many
applications, this input impedance drift can be ignored, and the values given in Table 1 for typical input
impedance are valid.
FULL-SCALE INPUT
A programmable gain amplifier (PGA) is implemented before the ADS1118 ΔΣ core. The PGA is configured by
three bits (PGA[2:0], bits 11-9) in the Config register and can be set to gains of 2/3, 1, 2, 4, 8, and 16. Table 2
shows the corresponding full-scale (FS) ranges. However, analog input voltages may never exceed the analog
input voltage limits given in the Electrical Characteristics table. In case a supply voltage of VDD greater than 4 V
is used, the 2/3 PGA setting allows input voltages to extend up to the supply. Note though that in this case, or
whenever the supply voltage is less than the FS range (for example, VDD = 3.3 V and PGA = 1), a full-scale
ADC output code cannot be obtained. This inability means that some dynamic range is lost.
Table 2. PGA Gain and Corresponding Full-Scale
Range
PGA SETTING
2/3
1
2
4
8
16
FS (V)
±6.144 ⁽¹⁾
±4.096 ⁽¹⁾
±2.048
±1.024
±0.512
±0.256
(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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