SPT
7
3/11/97
SPT7810
TYPICAL INTERFACE CIRCUIT
The SPT7810 requires few external components to achieve
the stated operation and performance. Figure 2 shows the
typical interface requirements when using the SPT7810 in
normal circuit operation.
The following section provides a description of the pin func-
tions and outlines critical performance criteria to consider for
achieving the optimal device performance.
POWER SUPPLIES AND GROUNDING
The SPT7810 requires the use of two supply voltages, VEE
and VCC. Both supplies should be treated as analog supply
sources. This means the VEE and VCC ground returns of the
device should both be connected to the analog ground
plane. All other -5.2 V requirements of the external digital
logic circuit should be connected to the digital ground plane.
Each power supply pin should be bypassed as closely as
possible to the device with .01
µF and 10 µF capacitors as
shown in figure 2.
The two grounds available on the SPT7810 are AGND and
DGND. DGND is used only for ECL outputs and is to be
referenced to the output pulldown voltage. These grounds
are not tied together internal to the device. The use of ground
planes is recommended to achieve the best performance of
the SPT7810. The AGND and the DGND ground planes
should be separated from each other and only connected
together at the device through an inductance. Doing this will
minimize the ground noise pickup.
VOLTAGE REFERENCE
The SPT7810 requires the use of two voltage references:
VFT and VFB. VFT is the force for the top of the voltage
reference ladder (+2.5 V typ), VFB (-2.5 V typ) is the force for
the bottom of the voltage reference ladder. Both voltages are
applied across an internal reference ladder resistance of 800
ohms. In addition, there are 3 reference ladder taps (VST,VRM
and VSB). VST is the sense for the top of the reference ladder
(+2.0 V), VRM is the midpoint of the ladder (0.0 V typ)
and VSB is the sense for the bottom of the reference
ladder (-2.0 V). The voltages seen at VST and VSB are the
true full scale input voltages of the device when VFT and VFB
are driven to the recommended voltages (+2.5 V and -2.5 V
typical respectively). These points should be used to monitor
the actual full scale input voltage of the device and should not
be driven to the expected ideal values as is commonly done
with standard flash converters. When not being used, a
decoupling capacitor of .01 uF connected to AGND from
each tap is recommended to minimize high frequency noise
injection.
An example of a reference driver circuit recommended is
shown in figure 2. IC1 is REF-03, the +2.5 V reference with a
Figure 2 - Typical Interface Circuit
VIN2
8
VRM
.01 µF
10 µF
R
2R
2R
2R
2R
R
4
Coarse
A/D
11 x 50
Ω
D0 (LSB)
D1
D2
D3
D4
D5
D6
D7
D8
D9 (MSB)
D10 (OVERRANGE)
-2 V
-5.2 V
+5 V
AGND
( 5 V RTN &
-5.2 V RTN )
DGND
( -2 V RTN )
VFT
VFB
SUCCESSIVE
INTERPOLATION
STAGE # N
SUCCESSIVE
INTERPOLATION
STAGE # i
T/H AMPLIFIER
BANK
ANALOG
PRESCALER
VIN1
+
10 µF
.01 µF
10 µF
.01 µF
+
+
L
.01 µF
1
-
2
3
4
6
7
2
VIN
VOUT
Trim
GND
R1
10 k
Ω
R2
*
30 k
Ω
R4
10 k
Ω
R3
*
30 k
Ω
IC1
IC2
(REF-03)
6
5
4
-5.2 V
.01 µF
+2.5 V
-5.2 V
+5 V
VST
.01 µF
VSB
*R2 and R3
matched to 0.1%
.01 µF
+
10 µH
+5 V
2
CLK
CLK
CLK-IN
CLK-IN
Analog
Input
Analog
Input
10 µF
+5 V
.01 µF
+
10 µF
-2.5 V
D2
D1
NOTE: D1=D2=1N5817 or equivalent. (Used to prevent damage caused by power sequencing.)
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