ADS8344

12

SBAS139E

REFERENCE INPUT

The external reference sets the analog input range. The

ADS8344 will operate with a reference in the range of

difference between the +IN input and the –IN input, as

shown in Figure 2. For example, in the single-ended mode,

a 1.25V reference with the COM pin grounded, the selected

input channel (CH0 - CH7) will properly digitize a signal in

the range of 0V to 1.25V. If the COM pin is connected to

0.5V, the input range on the selected channel is 0.5V to

1.75V.

There are several critical items concerning the reference

input and its wide-voltage range. As the reference voltage is

reduced, the analog voltage weight of each digital output

code is also reduced. This is often referred to as the LSB

FIGURE 2. Simplified Diagram of the Analog Input.

Converter

+IN

CH0

CH1

CH2

CH3

A2-A0

(shown 00o

(1)

SGL/DIF

(shown HIGH)

CH4

CH5

CH6

CH7

COM

NOTE: (1) See Truth Tables, Table I

and Table II for address coding.

(Least Significant Bit) size and is equal to the reference

voltage divided by 65536. Any offset or gain error inherent

in the A/D converter will appear to increase, in terms of LSB

size, as the reference voltage is reduced. For example, if the

offset of a given converter is 2LSBs with a 2.5V reference,

then it will typically be 10LSBs with a 0.5V reference. In

each case, the actual offset of the device is the same,

76.3

µV.

Likewise, the noise or uncertainty of the digitized output

will increase with lower LSB size. With a reference voltage

of 500mV, the LSB size is 7.6

µV. This level is below the

internal noise of the device. As a result, the digital output

code will not be stable and will vary around a mean value by

a number of LSBs. The distribution of output codes will be

gaussian and the noise can be reduced by simply averaging

consecutive conversion results or applying a digital filter.

With a lower reference voltage, care should be taken to

provide a clean layout including adequate bypassing, a clean

(low-noise, low-ripple) power supply, a low-noise reference,

and a low-noise input signal. Because the LSB size is lower,

the converter will also be more sensitive to nearby digital

signals and electromagnetic interference.

drives the Capacitor Digital-to-Analog Converter (CDAC)

portion of the ADS8344. Typically, the input current is

13

µA with a 2.5V reference. This value will vary by

microamps depending on the result of the conversion. The

reference current diminishes directly with both conversion

rate and reference voltage. As the current from the reference

is drawn on each bit decision, clocking the converter more

quickly during a given conversion period will not reduce

overall current drain from the reference.

DIGITAL INTERFACE

The ADS8344 has a four-wire serial interface compatible

with several microprocessor families (note that the digital

inputs are over-voltage tolerant up to +5.5V, regardless of

digital interface.

Most microprocessors communicate using 8-bit transfers;

the ADS8344 can complete a conversion with three such

transfers, for a total of 24 clock cycles on the DCLK input,

provided the timing is as shown in Figure 3.

FIGURE 3. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK Delay Required with Dedicated

Serial Port.

t

ACQ

Acquire

Idle

Conversion

1

DCLK

CS

81

15

BUSY

(MSB)

(START)

(LSB)

A2

S

A1

A0

SGL/

DIF

PD1 PD0

14

13

12

11

10

9

8

7654321

0

Zero Filled...

81

8

Acquire

Idle

Conversion

18

1

15

(MSB)

(START)

A2

SA1

A0

SGL/

DIF

PD1 PD0

14