Figure 6 – On-Chip Protection Circuit
V
DD
120
Ω
DIGITAL OUTPUTS
Analog
120
Ω
Pad
The digital outputs (D0–D12) are driven by a separate sup-
ply (OV
DD
) ranging from +3 V to +5 V. This feature makes it
possible to drive the SPT7938’s TTL/CMOS-compatible
outputs with the user’s logic system supply. The format of
the output data (D0–D11) is straight binary. (See table III.)
The outputs are latched on the rising edge of CLK. These
outputs can be switched into a tri-state mode by bringing
EN
high.
Table III – Output Data Information
ANALOG INPUT
+F.S. + 1/2 LSB
+F.S. –1/2 LSB
+1/2 F.S.
+1/2 LSB
0.0 V
OVERRANGE
D12
1
0
0
0
0
OUTPUT CODE
D11–D0
11 1111 1111
11 1111 111Ø
ØØ ØØØØ ØØØØ
00 0000 000Ø
00 0000 0000
INPUT PROTECTION
All I/O pads are protected with an on-chip protection circuit
shown in figure 6. This circuit provides ESD robustness to
3.5 kV and prevents latch-up under severe discharge condi-
tions without degrading analog transition times.
(Ø indicates the flickering bit between logic 0 and 1).
OVERRANGE OUTPUT
The Overrange Output (D12) is an indication that the analog
input signal has exceeded the positive full-scale input volt-
age by 1 LSB. When this condition occurs, D12 will switch to
logic 1. All other data outputs (D0 to D11) will remain at
logic 1 as long as D12 remains at logic 1. This feature
makes it possible to include the SPT7938 in higher resolu-
tion systems.
CLOCK INPUT
The SPT7938 is driven from a single-ended TTL-input
clock. The duty cycle of the clock should be kept as close to
50% (±5%) as possible.
EVALUATION BOARD
The EB7938 evaluation board is available to aid designers
in demonstrating the full performance of the SPT7938. This
board includes a reference circuit, clock driver circuit, output
data latches and an on-board reconstruction of the digital
data. An application note (AN7938) describing the operation
of this board, as well as information on the testing of the
SPT7938, is also available. Contact the factory for price and
availability.
SPT7938
8
5/24/00
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
