ADS1254

7

SBAS213

THEORY OF OPERATION

The ADS1254 is a precision, high-dynamic range, 24-bit,

delta-sigma, A/D converter capable of achieving very

high-resolution digital results at high data rates.

The analog-input signal is sampled at a rate determined by the

frequency of the system clock (CLK). The sampled analog

input is modulated by the delta-sigma A/D modulator, which

processes the output of the delta-sigma modulator and writes

the result into the data-output register. The DOUT/DRDY pin

is pulled LOW, indicating that new data is available to be read

by the external microcontroller/microprocessor. As shown in

the block diagram, the main functional blocks of the ADS1254

are the fourth-order delta-sigma modulator, a digital filter,

control logic, and a serial interface. Each of these functional

blocks is described below.

ANALOG INPUT

The ADS1254 contains a fully differential analog input. In order

to provide low system noise, common-mode rejection of 102dB,

and excellent power-supply rejection, the design topology is

based on a fully differential switched-capacitor architecture. The

bipolar input voltage range is from –4.096V to +4.096V, when

the reference input voltage equals +4.096V. The bipolar range is

Figure 1 shows the basic input structure of the ADS1254.

The impedance is directly related to the sampling frequency

of the input capacitor that is set by the CLK rate. Higher

CLK rates result in lower impedance, and lower CLK rates

result in higher impedance.

20.8kHz with a –3dB frequency of 4.24kHz. The –3dB

frequency scales with the system clock frequency.

To ensure the best linearity of the ADS1254, a fully differen-

tial signal is recommended.

INPUT MULTIPLEXER

The CHSEL1 and CHSEL0 pins are used to select the analog

input channel, as shown in Table I. The recommended

method for changing channels is to change them after the

conversion from the previous channel has been completed

and read. When a channel is changed, internal logic senses

the change on the falling edge of CLK and resets the

conversion process. The conversion data from the new

channel is valid on the first DRDY after the channel change.

When multiplexing inputs, it is possible to achieve sample

rates close to 4kHz. This is due to the fact that it requires five

internal conversion cycles for the data to fully settle. The

data also must be read before the channel is changed. The

DRDY signal indicates a valid result after the five cycles

have occurred.

BIPOLAR INPUT

R

SW

(1300

Ω typical)

Modulator Frequency

= f

MOD

Internal

Circuitry

C

INT

(6pF typical)

V

CM

A

IN

The input impedance of the analog input changes with the

ADS1254 system clock frequency (CLK). The relationship is:

With regard to the analog-input signal, the overall analog

performance of the device is affected by three items: first, the

input impedance can affect accuracy. If the source impedance

of the input signal is significant, or if there is passive filtering

prior to the ADS1254, a significant portion of the signal can be

lost across this external impedance. The magnitude of the

effect is dependent on the desired system performance.

Second, the current into or out of the analog inputs must be

limited. Under no conditions should the current into or out

of the analog inputs exceed 10mA.

Third, to prevent aliasing of the input signal, the analog-input

signal must be band limited. The bandwidth of the A/D

converter is a function of the system clock frequency. With a

system clock frequency of 8MHz, the data-output rate is

FIGURE 1. Analog-Input Structure.

FIGURE 2. Level Shift Circuit for Bipolar Input Ranges.

BIPOLAR INPUT

±10V

2.5k

Ω

5k

Ω

±5V

5k

Ω

10k

Ω

±2.5V

10k

Ω

20k

Ω

10k

Ω

20k

Ω

R

1

OPA4350

OPA4350

OPA4350

+IN

–IN

V

REF

ADS1254

Bipolar

Input

REF

2.5V

CHSEL1

CHSEL0

CHANNEL

0

0

CH1

0

1

CH2

1

0

CH3

1

1

CH4

TABLE I. Channel Selection.

Each of the differential inputs of the ADS1254 must stay

voltage, and the other input can range from AGND to

amplifier and four external resistors, the ADS1254 can be

configured to accept bipolar inputs referenced to ground.

The conventional

±2.5V, ±5V, and ±10V input ranges can

be interfaced to the ADS1254 using the resistor values

shown in Figure 2.