ADS8345EB datasheet(10/26 Pages) TI2

10 / 26 page

ADS8345

SBAS177C

www.ti.com

THEORY OF OPERATION

The ADS8345 is a classic Successive Approximation

capacitive redistribution which inherently includes a sample-

and-hold function. The converter is fabricated on a 0.6

µm

CMOS process.

The basic operation of the ADS8345 is shown in Figure 1.

The device requires an external reference and an external

clock. It operates from a single supply of 2.7V to 5.25V. The

external reference can be any voltage between 500mV and

+VCC/2. The value of the reference voltage directly sets the

input range of the converter. The average reference input

current depends on the conversion rate of the ADS8345.

The analog input to the converter is differential and is

provided via an eight-channel multiplexer. The input can be

provided in reference to a voltage on the COM pin (which is

generally +VCC/2) or differentially by using four of the eight

input channels (CH-CH7). The particular configuration is

selectable via the digital interface.

ANALOG INPUT

The analog input is bipolar and fully differential. There are

two general methods of driving the analog input of the

ADS8345: single-ended or differential (see Figure 2). When

the input is single-ended, the COM input is held at a fixed

voltage. The CHX input swings around the same voltage and

the peak-to-peak amplitude is 2 • VREF. The value of VREF

FIGURE 1. Basic Operation of the ADS8345.

determines the range over which the common voltage may

vary (see Figure 3).

When the input is differential, the amplitude of the input is the

difference between the CHX and COM input (see Figure 4).

A voltage or signal is common to both of these inputs. The

peak-to-peak amplitude of each input is VREF about this

common voltage. However, since the input are 180

°C out-of-

phase, the peak-to-peak amplitude of the difference voltage is

2 • VREF. The value of VREF also determines the range of the

voltage that may be common to both inputs (see Figure 5).

In each case, care should be taken to ensure that the output

impedance of the sources driving the CHX and COM inputs

are matched. If this is not observed, the two inputs could

have different settling times. This may result in offset error,

gain error, and linearity error which changes with both

temperature and input voltage. If the impedance cannot be

matched, the errors can be lessened by giving the ADS8345

additional acquisition time.

The input current on the analog inputs depends on a number

of factors: sample rate, input voltage, and source impedance.

Essentially, the current into the ADS8345 charges the inter-

nal capacitor array during the sample period. After this

capacitance has been fully charged, there is no further input

current.

Care must be taken regarding the absolute analog input

voltage. Outside of these ranges, the converter’s linearity

may not meet specifications. Please refer to the Electrical

Characteristics table for min/max ratings.

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

SHDN

DCLK

BUSY

OUT

GND

REF

Serial/Conversion Clock

Chip Select

Serial Data In

Serial Data Out

+2.7V to +5V

µF to 10µF

ADS8345

Single-ended

or differential

analog inputs.

REF

µF to 10µF

+1.25V to +2.5V

0.1

µF

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