Figure 1 – Typical Interface Circuit
T1
0.1
+
10
V
CM
V
IN
R
T1
50
W
Ext
+A5
+
10
0.1
+
10
0.1
A
IN
SPT7864
V
IN
OGND
OV
DD
GND
CLK
0.1
0.1
R
T2
50
W
CLK
V
DD
Mini-Circuits
T16T or
T11T
EXT/INT
REF
L
REF
H
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
(MSB)
(LSB)
+D3/5
50
W
Buffer
+ 10
+ 10
+A5
+A3/5
AGND
AGND
0.1
10
+D3/5
DGND
+
Ferrite Bead
+A5
T2
CLK
IN
TYPICAL INTERFACE CIRCUIT
REFERENCES
The SPT7864 has a differential analog input. The input
range is determined by the voltages V
IN
and V
IN
applied to
reference pins REF
H
and REF
L
respectively, and is equal
to ±(V
IN
–V
IN
). Externally generated reference voltages
connected to REF
H
and REF
L
should be symmetric
around 2.5 V. The input range can be defined between
±0.6 V and ±1.5 V. An internal reference exists, providing
reference voltages at pins REF
H
and REF
L
equal to +3.0 V
(V
REFH
) and +2.0 V (V
REFL
). These can be connected to
REF
H
and REF
L
by connecting pin EXT/
INT
to GND. The
references should be bypassed as close to the converter
pins as possible using 100 nF capacitors in parallel with
smaller capacitors (e.g. 220 pF) to ground.
ANALOG INPUT
The input of the SPT7864 can be configured in various
ways, dependent upon whether a single-ended or differen-
tial, AC- or DC-coupled input is wanted.
AC-coupled input is most conveniently implemented using
a transformer with a center-tapped secondary winding.
The center tap is connected to the V
CM
node, as shown in
figure 1. In order to obtain low distortion, it is important that
the selected transformer does not exhibit core saturation
at full scale. Excellent results are obtained with the Mini-
Circuits T1-6T or T1-1T. Proper termination of the input is
important for input signal purity. A small capacitor (typically
68 pF) across the inputs attenuates kickback noise from
the sample-and-hold. A small capacitor (1 nF) between
V
CM
and ground has also been proven to be advanta-
geous.
If a DC-coupled, single-ended input is wanted, a solution
based on operational amplifiers, as shown in figure 2, is
usually preferred. The AD8138 is suggested for low distor-
tion and video bandwidth. Lower cost operational amplifi-
ers may be used if the demands are less strict.
Figure 2 – DC-Coupled, Single-Ended to Differential
Conversion
(power supplies and bypassing
not shown)
51
W
470
W
Input
Offset
Analog
In
AD8138
470
W
100
W
AD8138
Mini-Circuits
T16T or
T11T
1 kW
1 kW
51
W
15 pF
100
W
51
W
470
W
470
W
AD8138
51
W
470
W
SPT7864
5
11/20/01
Logic Interface Circuit
ADC
V
IN
V
IN
Sleep
Sleep
OR
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
