initialization phase until RDY is deasserted. Note that,
although typically the device is initialized when power is
first applied, the initialization is only started when the
RS
is
asserted; there is no “power-on-reset” circuitry on chip.
RS
may be held low for an indefinite period of time. While
RS
is
low, RDY will remain high. After
RS
is returned to high, RDY
will go low for the duration of the calibration.
TYPICAL INTERFACE CIRCUIT
ANALOG INPUT DRIVER
The differential analog inputs (V
IN
+, V
IN
–) have a resistive
input impedance of 1 kΩ minimum. For best performance,
the input source should be a differential input, as shown in
figure 2, typical interface circuit. The SPT8100 provides its
own common-mode voltage on the pin marked V
CM
. Out-
put drive capability of V
CM
is a maximum of 47 µA (50 kΩ to
ground).
The SPT8100 application note (AN8100) shows an ex-
ample of two modes of driving the SPT8100. One mode is
through a transformer and the other is through a single-to-
differential converter. In all cases, both inputs V
IN
+ and
V
IN
– must be kept within the input common-mode range
(1.15 V to 3.65 V).
BIAS
C
CONNECTION
An external capacitor, C
EXT
on the BIAS
C
pin, is used only
for noise filtering of an internal voltage associated with the
references. Its value is not critical: 1 µF in parallel with
0.01 µF is recommended.
BIAS
R
CONNECTION
As shown in the typical interface circuit, R
EXT
is needed to
connect between BIAS
R
to ground. This resistor ranges
from 800
Ω
to 2.5 kΩ. The proper selection of R
EXT
is a
function of the sample rate and input frequency. Nominally,
at 5 MSPS, R
EXT
=1.43 kΩ is recommended. If linearity for
large signal levels at an analog bandwidth of 2 MHz is criti-
cal, the value should be decreased to R
EXT
=1.24 kΩ; and
for even higher-frequency analog inputs, R
EXT
=1.0 kΩ can
be used. At lower sample rates (for example 2 MSPS),
and lower analog input frequencies, the value may be in-
creased to R
EXT
=2 kΩ. (Refer to the typical interface circuit
table in figure 2b.)
PROGRAMMABLE GAIN AMPLIFIER
The programmable gain amplifier (PGA) precedes the
ADC inputs. The differential inputs, which are resistive, are
at pins V
IN
+ and V
IN
–.The maximum input range is 5 V
peak-to-peak differential (2.5 V single-ended). To achieve
maximum overall system noise performance, the source
driving these inputs needs to be as low-noise and as low-
jitter as possible, while maintaining the required distortion
performance. In addition, the driving source must be low
impedance to maintain the accuracy of the PGA gain.
The internal 0 dB analog signal level and ADC full-scale
output level is 5 V peak-to-peak differential (2.5 V single-
ended). The PGA may be used to provide gain for an input
less than 5 V peak-to-peak differential.
The gain of the PGA can be programmed using a three-bit
control, available at pins GS0 to GS2. See table I. Note that
the input resistance is a function of the gain setting.
Table I – PGA Gain Control
GS2 GS1 GS0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
PGA
Input
V/V
Gain Resistance Gain
Ω
(dB)
(kΩ)
0
2.9
5.8
11.8
14.8
17.5
19.5
X
5.57
4.65
3.97
2.23
1.66
1.25
1.00
1
1.40
1.95
3.9
5.5
7.5
9.5
Forbidden
3 dB
BW LSB
RMS
12
10
8
7
6
5.5
5
1.4
1.5
1.6
2.0
2.3
2.6
2.8
POWER SUPPLIES AND GROUNDING
The SPT8100 requires three power supplies: analog AV
DD
,
digital DV
DD
and output supply OV
DD
. This device works
best if all three supplies are coming from the analog supply
side of the system as shown in the typical interface circuit
(figure 2a).
Note, in figure 2a, that the supplies to the logic interface
circuit and the OV
DD
are separate from each other. In a
case where the +A3.3/5 V supply is not available, try to
implement the design as close as possible to that shown
in figure 2b. Place the ferrite bead (FB1) as close to the
device as possible. To avoid latch-up, the delta between
all three grounds must stay with 100 mV; this includes
transients. (Refer to the absolute maximum ratings
specifications.)
SPT8100
5
1/9/02
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
