Datasheet

ADS901
9
SBAS054A
R
1
1kΩ
OPA680
V
IN
R
F
402Ω
V
CM
C
1
0.1µF
0.1µF
IN
CM
+5V
R
S
50Ω
+3V
R
G
402Ω
C
G
0.1µF
402Ω
22pF
ADS901
FIGURE 4. DC-Coupled Interface Circuit for +3V Single-Supply Operation.
dual supplies (OPA650, OPA658). The mid-point reference
voltage, V
CM
, biases the bipolar, ground-referenced input
signal. The capacitor C
1
and resistor R
1
form a high-pass
filter with the –3dB frequency set at
f
–3dB
= 1/(2 π R
1
C
1
) (2)
The values for C
1
and R
1
are not critical in most applications
and can be set freely. The values shown correspond to a
frequency of 1.6kHz.
Figure 3 depicts a circuit that can be used in single-supply
applications. The mid-reference voltage biases the op amp
up to the appropriate common-mode voltage, for example
V
CM
= +1.5V. With the use of capacitor C
G
the DC gain for
the non-inverting op amp input is set to +1V/V. As a result
the transfer function is modified to
V
OUT
= V
IN
{(1 + R
F
/R
G
) + V
CM
} (3)
Again, the input coupling capacitor C
1
and resistor R
1
form
a high-pass filter. At the same time the input impedance is
defined by R
1
. Resistor R
S
isolates the op amp’s output from
the capacitive load to avoid gain peaking or even oscillation.
It can also be used to establish a defined bandwidth to reduce
the wideband noise. The recommended value is usually
between 10Ω and 100Ω.
DC-COUPLED INTERFACE CIRCUIT
Many systems are now requiring +3V single supply capabil-
ity of both the A/D converter and its driver. Figure 4 shows
an example for DC-coupled configuration operating solely
on a +3V supply voltage. The OPA632 provides excellent
performance in this demanding application. Its wide input
and output voltage ranges, an low distortion, supports the
ADS901 well. The OPA632 is configured for a gain of +2.
The 374Ω and 2.26kΩ resistors at the input level-shift V
IN
so that V
OUT
is within the allowed output voltage range
when V
IN
= 0. The input impedance of the driver circuit is
set to match to a 50Ω source impedance. The input level-
shifting was designed that V
IN
can be between 0V and 5V,
while delivering an output voltage of 1V to 2V into the
ADS901. Both the OPA632 and ADS901 have a power-
down function pin with the same polarity for those systems
the need to conserve power.
EXTERNAL REFERENCE
The ADS901 requires external references on pin 22 (REFT)
and 24 (REFB). Internally those pins are connected through a
resistor ladder, which has a nominal resistance of 4kΩ
(±15%). In order to establish a correct voltage drop across the
ladder the external reference circuit must be able to typically
supply 250µA of current. With this current the full-scale input
range of the ADS901 is set between +1V and +2V, or 1Vp-p.
In general, the voltage drop across REFT and REFB deter-
mines the input full-scale range (FSR) of the ADS901. Equa-
tion (4) can be used to calculate the span.
FSR = REFT - REFB (4)
Depending on the application, several options are possible to
supply the external reference voltages to the ADS901 without
degrading the typical performance.
FIGURE 3. Interface Circuit. Example using the voltage feedback amplifier OPA680.
OPA632
V
IN
750Ω562Ω
2.26kΩ
374Ω
22pF
+3V
100Ω
Pwrdn
DIS
Disable
+3V
ADS901
10-Bit
20Msps
57.6Ω