8

®

ADS804

APPLICATION INFORMATION

DRIVING THE ANALOG INPUT

The ADS804 allows its analog inputs to be driven either

single-ended or differentially. The focus of the following

discussion is on the single-ended configuration. Typically,

its implementation is easier to achieve and the rated speci-

fications for the ADS804 are characterized using the single-

ended mode of operation.

AC-COUPLED INPUT CONFIGURATION

Given in Figure 1 is the circuit example of the most common

connected to the SEL pin, the full-scale input range is

defined to be 2Vp-p. This signal is ac-coupled in single-

ended form to the ADS804 using the low distortion voltage-

feedback amplifier OPA642. As is generally necessary for

single supply components, operating the ADS804 with a

full-scale input signal swing requires a level-shift of the

amplifier’s zero centered analog signal to comply with the

A/D converters input range requirements. Using a DC block-

ing capacitor between the output of the driving amplifier and

the converter’s input, a simple level-shifting scheme can be

implemented. In this configuration, the top and bottom

references (REFT, REFB) provide an output voltage of +3V

and +2V, respectively. Here, two resistor pairs (2 x 2k

Ω) are

used to create a common-mode voltage of approximately

+2.5V to bias the inputs of the ADS804 (IN, IN) to the

required DC voltage.

An advantage of ac-coupling is that the driving amplifier

still operates with a ground-based signal swing. This will

keep the distortion performance at its optimum since the

signal swing stays within the linear region of the op amp and

sufficient headroom to the supply rails can be maintained.

Consider using the inverting gain configuration to eliminate

CMR induced errors of the amplifier. The addition of a small

input of the ADS804 will be beneficial in almost all interface

configurations. This will decouple the op amp’s output from

the capacitive load and avoid gain peaking, which can result

in increased noise. For best spurious and distortion perfor-

mance, the resistor value should be kept below 100

Ω.

Furthermore, the series resistor together with the 100pF

capacitor establish a passive low-pass filter, limiting the

bandwidth for the wideband noise thus, help improving the

SNR performance.

DC-COUPLED WITHOUT LEVEL SHIFT

In some applications the analog input signal may already be

biased at a level which complies with the selected input

range and reference level of the ADS804. In this case, it is

only necessary to provide an adequately low source imped-

ance to the selected input, IN or IN. Always consider

wideband op amps since their output impedance will stay

low over a wide range of frequencies. For those applications

requiring the driving amplifier to provide a signal amplifica-

tion, with a gain

≥ 3, consider using the decompensated

voltage feedback op amp OPA643.

DC-COUPLED WITH LEVEL SHIFT

Several applications may require that the bandwidth of the

signal path include DC, in which case the signal has to be

DC-coupled to the A/D converter. In order to accomplish

this, the interface circuit has to provide a DC-level shift. The

circuit shown in Figure 2 employs an op amp, A1, to sum the

ground centered input signal with a required DC offset. The

ADS804 typically operates with a +2.5V common-mode

voltage, which is established at the center tap of the ladder

and connected to the IN input of the converter. Amplifier A1

set the DC-bias level for A1. Because of the op amp’s noise

applied to its non-inverting input has to be divided down to

+1.25V, resulting in a DC output voltage of +2.5V.

FIGURE 1. AC-Coupled Input Configuration for 2Vp-p Input Swing and Common-Mode Voltage at +2.5V Derived from

Internal Top and Bottom Reference.

OPA642

V

IN

+V

IN

0V

–V

IN

R

F

402

Ω

R

G

402

Ω

ADS804

R

S

24.9

Ω

2k

Ω

2k

Ω

2k

Ω

2k

Ω

100pF

0.1µF

0.1

µF

2Vp-p

IN

IN

(+2V)

REFB

(+1V)

VREF

SEL

REFT

(+3V)

+5V –5V

+2.5V

DC