ADS808 Datasheet, PDF (11/19 Pages) Burr-Brown (TI) – 12-Bit, 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER

attenuate some of the wideband noise. The actual compo-
nent values would need to be tuned for the individual appli-
cation requirements. As a guideline, resistor values are
typically in the range of 10Ω to 100Ω, capacitors in the range
of 10pF to 200pF. In any case, the R_IN and C_IN values should
have a low tolerance. This will ensure that the ADS808 sees
closely matched source impedances.
AC-Coupled, Differential Interface with Gain
The interface circuit example presented in Figure 6 employs
two OPA685s, (current-feedback op amps), optimized for
gains of 8V/V or higher. The input transformer (T1) converts
the single-ended input signal to a differential signal required
at the amplifier’s inverting inputs, that are tuned to provide a
50Ω impedance match to an assumed 50Ω source. To
achieve the 50Ω input match at the primary of the 1:2
transformer, the secondary input must see a 200Ω load
impedance. Both amplifiers are configured for the inverting
mode resulting in close gain and phase matching of the
differential signal. This technique, along with a highly sym-
metrical layout, is instrumental in achieving a substantial
reduction of the 2nd-harmonic, while retaining excellent 3rd-
order performance. A common-mode voltage (V_CM) is ap-
plied to the noninverting inputs of the OPA685. Additional
series of 43.2Ω resistors isolate the output of the op amps
from the capacitive load presented by the 22pF capacitors
and the input capacitance of the ADS808. This 43.2Ω/22pF
combination sets a pole at approximately 167MHz and rolls
off some of the wideband noise.
REFERENCE
REFERENCE OPERATION
Integrated into the ADS808 is a bandgap reference circuit
including some logic that provides a +0.5V, +0.75V, or +1V
reference output by selecting the corresponding pin-strap
configuration. Table I gives a complete overview of the
possible reference options and pin configurations.
V_CM
50Ω Source
T1
V_I
1:2
Noise
Figure
11.8dB
100Ω
100Ω
V_CM
+5V
DIS
OPA685
–5V
600Ω
600Ω
+5V
DIS
OPA685
–5V
Power-supply decoupling
not shown.
43.2Ω
43.2Ω
22pF
V_O A/D Converter Input
22pF
V_{O = 12V/V (21.6dB)}
V_I
FIGURE 6. Wideband Differential A/D Converter Driver.
DESIRED FULL-SCALE RANGE,
FSR (Differential)
CONNECT SEL1 CONNECT SEL2
(Pin 33)
(Pin 32)
VOLTAGE AT V_REF
(Pin 34)
VOLTAGE AT REFT
(Pin 41)
2Vp-p (+10dBm)
1.5Vp-p (+7.5dBm)
1Vp-p
External Reference
GND
GND
V_REF
—
GND
+V_S
GND
—
+1.0V
+0.75V
+0.5V
> +3.5V
+3V
+2.875V
+2.75V
+2.75V to +4.5V
TABLE I. Reference Pin Configurations and Corresponding Voltage on the Reference Pins.
VOLTAGE AT REFB
(Pin 39)
+2V
+2.125V
+2.25V
+0.5V to +2.25V
ADS808
11
SBAS179C
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