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ADS805U1K Datasheet(PDF) 8 Page - Texas Instruments

Part # ADS805U1K
Description  12-Bit, 20MHz Sampling ANALOG-TO-DIGITAL CONVERTER
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

ADS805U1K Datasheet(HTML) 8 Page - Texas Instruments

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ADS805
8
SBAS073B
www.ti.com
APPLICATION INFORMATION
DRIVING THE ANALOG INPUT
The ADS805 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 ADS805 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 ADS805. With the VREF 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 ADS805 using the low distortion voltage-feedback
amplifier OPA642. As is generally necessary for single-
supply components, operating the ADS805 with a full-scale
input signal swing requires a level-shift of the amplifier’s zero
centered analog signal to comply with the ADC’s 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, respectively.
Here, two resistor pairs (2 • 2k
Ω) are used to create a
common-mode voltage of approximately +2.5V to bias the
inputs of the ADS805 (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 (RS) between the output of the op amp and the
input of the ADS805 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 helping improve the
signal-to-noise 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 ADS805. 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.
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 ADC. In order to accomplish this, the interface
circuit has to provide a DC-level shift. The circuit presented in
Figure 2 utilizes the single-supply, current-feedback op amp
OPA681 (A1), to sum the ground-centered input signal with a
required DC offset. The ADS805 typically operates with a
+2.5V common-mode voltage, which is established with resis-
tors R3 and R4 and connected to the IN input of the converter.
Amplifier A1 operates in inverting configuration. Here, resistors
R1 and R2 set the DC-bias level for A1. Because of the op
amp’s noise gain of +2V/V, assuming RF = RIN, 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. DC
voltage differences between the IN and IN inputs of the
ADS805 effectively will produce an offset, which can be cor-
rected for by adjusting the values of resistors R1 and R2. The
bias current of the op amp may also result in an undesired
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
R
G
402
ADS805
R
S
24.9
2k
2k
2k
2k
+2.5V
100pF
0.1
µF
0.1
µF
2Vp-p
+5V –5V
IN
IN
(+2V)
REFB
(+1V)
V
REF
SEL
REFT
(+3V)


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