ADS803

8

SBAS074B

www.ti.com

FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-Level Shift.

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 in-

duced errors of the amplifier. The addition of a small series

of the ADS803 will be beneficial in almost all interface configu-

rations. 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 performance,

the resistor value should be kept below 50

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 ADS803. 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 amplification (with a

gain

≥ 3), consider using the decompensated voltage-feed-

back op amp OPA643.

DC-COUPLED WITH LEVEL SHIFT

Several applications may require that the bandwidth of the

signal path includes 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 presented in Figure 2 employs an op amp, A1, to sum

the ground centered input signal with a required DC offset.

OPA691

V

IN

+V

S

2Vp-p

0

+1V

–1V

R

F

R

IN

NOTE: R

+V

S

ADS803

R

S

24.9

Ω

2k

Ω

100pF

10

µF

+

+2.5V

R

2

R

1

2k

Ω

IN

IN

REFB

(+1V)

V

REF

SEL

REFT

0.1

µF

0.1

µF

The ADS803 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

applied to its noninverting input has to be divided down to

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

DC voltage differences between the IN and IN inputs of the

ADS803 will effectively produce an offset, which can be

The bias current of the op amp may also result in an

undesired offset. The selection criteria of the appropriate op

amp should include the input bias current, output voltage

swing, distortion, and noise specification. Note that in this

example the overall signal phase is inverted. To re-estab-

lish the original signal polarity it is always possible to

interchange the IN and IN connections.

SINGLE-ENDED-TO-DIFFERENTIAL

CONFIGURATION (TRANSFORMER COUPLED)

In order to select the best suited interface circuit for the

ADS803, the performance requirements must be known. If

an ac-coupled input is needed for a particular application, the

next step is to determine the method of applying the signal;

either single-ended or differentially. The differential input

configuration may provide a noticeable advantage of achiev-

ing good SFDR performance based on the fact that in the

differential mode, the signal swing can be reduced to half of

the swing required for single-ended drive. Secondly, by

driving the ADS803 differentially, the even-order harmonics

will be reduced. See Figure 3 for the schematic of the

suggested transformer coupled interface circuit. The resistor