TRANSITION NOISE
DESCRIPTION
ANALOG INPUT
(1)
DIGITAL OUTPUT
BINARY TWO’S COMPLEMENT
Figure 5 shows a histogram plot for the ADS7861 following
8,000 conversions of a DC input. The DC input was set at
output code 2046. All but one of the conversions had an
output code result of 2046 (one of the conversions resulted
in an output of 2047). The histogram reveals the excellent
noise performance of the ADS7861.
Full-Scale Input Span
–VREF to +VREF
Least Significant
Bit (LSB)
(–VREF to +VREF)/4096(2)
BINARY CODE
HEX CODE
+Full Scale
Midscale
4.99878V
2.5V
0111 1111 1111
0000 0000 0000
1111 1111 1111
1000 0000 0000
7FF
000
FFF
800
Midscale – 1 LSB
–Full Scale
2.49878V
0V
NOTES: (1) –VREF to +VREF around VREF. With a 2.5V reference, this corre-
sponds to a 0V to 5V input span. (2) 1.22mV with a 2.5V reference.
BIPOLAR INPUTS
The differential inputs of the ADS7861 were designed to
accept bipolar inputs (–VREF and +VREF) around the internal
reference voltage (2.5V), which corresponds to a 0V to 5V
input range with a 2.5V reference. By using a simple op amp
circuit featuring a single amplifier and four external resis-
tors, the ADS7861 can be configured to except bipolar
inputs. The conventional ±2.5V, ±5V, and ±10V input
ranges can be interfaced to the ADS7861 using the resistor
values shown in Figure 7.
TABLE I. Ideal Input Voltages and Output Codes.
TIMING AND CONTROL
The operation of the ADS7861 can be configured in four
different modes by using the address pins M0, M1 and A0.
The M0 pin selects between two- and four-channel operation
(in two-channel operation, the A0 pin selects between Chan-
nels 0 and 1; in four-channel operation the A0 pin is ignored
and the channels are switched automatically after each
conversion). The M1 pin selects between having serial data
transmitted simultaneously on both the Serial A data output
and the Serial B data output or having both channels output
data through the Serial A port. The A0 pin selects either
Channel 0 or Channel 1 (see Pin Descriptions and Serial
Output Truth Table for more information).
8000
7000
6000
5000
4000
3000
2000
1000
0
The next four sections will explain the four different modes
of operation.
Mode I (M0 = 0, M1 = 0)
With the M0 and M1 pins both set to ‘0’, the ADS7861 will
operate in two-channel operation (the A0 pin must be used
to switch between Channels A and B). A conversion is
initiated by bringing CONVST HIGH for a minimum of
15ns. It is very important that CONVST be brought HIGH
a minimum of 10ns prior to a rising edge of the external
clock or 5ns after the rising edge. If CONVST is brought
2044
2045
2046
2047
2048
Code (decimal)
FIGURE 5. Histogram of 8,000 Conversions of a DC Input.
R1
1.4V
4kΩ
+IN
OPA132
20kΩ
3kΩ
Bipolar Input
–IN
DATA
Test Point
ADS7861
100pF
CLOAD
R2
REFOUT
2.5V
BIPOLAR INPUT
R1
R2
VOH
DATA
±10V
±5V
±2.5V
1kΩ
2kΩ
4kΩ
5kΩ
10kΩ
20kΩ
VOL
tR
tF
Voltage Waveforms for DATA Rise and Fall Times tR, and tF.
FIGURE 7. Level Shift Circuit for Bipolar Input Ranges.
FIGURE 6. Test Circuits for Timing Specifications.
ADS7861
10
SBAS110D
TI [ TEXAS INSTRUMENTS ]
