Datasheet

ADS804
8
SBAS068B
www.ti.com
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 specifica-
tions 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
interface configuration for the ADS804. With the V
REF
pin
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 convert-
ers input range requirements. Using a DC blocking 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, respec-
tively. Here, two resistor pairs (2 • 2kΩ) 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
series resistor (R
S
) between the output of the op amp and the
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 100Ω.
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 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 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. See the
circuit of Figure 2 which employs an op amp, to sum the
ground-centered input signal with a required DC offset. The
ADS804 typically operates with a +2.5V common-mode volt-
age, which is established at the center tap of the ladder and
connected to the
IN input of the converter. Amplifier A1
operates in inverting configuration. Here resistors R
1
and
R
2
set the DC-bias level for A1. Because of the op amp’s
noise gain of +2V/V, assuming R
F
= R
IN
, the DC offset
voltage applied to its noninverting 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
402Ω
R
G
402Ω
ADS804
R
S
24.9Ω
2kΩ2kΩ
2kΩ
2kΩ
100pF
0.1µF
0.1µF2Vp-p
IN
IN
(+2V)
REFB
(+1V)
VREF
SEL
REFT
(+3V)
+5V –5V
+2.5V
DC