INTERNAL CLOCK OPERATION
Figure 12b shows how to use the internal clock generating
circuitry. The clock frequency depends only on the value of
the resistor, as shown in “Internal Clock Frequency vs
RCLOCK” in the Typical Performance Curves section.
50Ω
Analog
Input
To ADS7832
5nF
REF– (Normally 0V)
The clock generator can operate between 100kHz and 2MHz.
With R = 100kΩ, the clock frequency will nominally be
800kHz. The internal clock oscillators may vary by up to
20% from device to device, and will vary with temperature,
as shown in the typical performance curves. Therefore, use
of an external clock source is preferred in applications where
control of the conversion timing is critical, or where multiple
converters need to be synchronized.
V
(a) Passive Low Pass Filter
To ADS7832
OPA627
R
Analog
Input
C
V
REF– (Normally 0V)
(b) Active Low Pass Filter
74HC-Compatible
Clock Source
CLK
To ADS7832
Pin 23
FIGURE 10. Input Signal Conditioning.
(a) External Clock Operation
See Figure 15 for typical
R Values vs Frequency
R
To ADS7832
Pin 23
+5V
(b) Internal Clock Operation
+5V
1
2
3
4
5
6
7
8
9
SFR
AIN0
AIN1
AIN2
AIN3
VA
28
10nF
+
FIGURE 12. Internal Clock Operation.
AGND 27
CAL 26
A1 25
10µF
5V
REF
APPLICATIONS
BIPOLAR INPUT RANGES
A0 24
Figure 13 shows a circuit to accurately and simply convert
a bipolar ±5V input signal into a unipolar 0 to 5V signal for
conversion by the ADS7832, using a precision, low-cost
complete difference amplifier, INA105.
VREF+
CLK 23
BUSY 22
HBE 21
WR 20
CS 19
RD 18
D0 17
10µF
+
+
10nF
10nF
VREF–
DGND
VD
10µF
Figure 14 shows a circuit to convert a bipolar ±10V input
signal into a unipolar 0 to 5V signal for conversion by the
ADS7832. The precision of this circuit will depend on the
matching and tracking of the three resistors used.
10 D7
11 D6
12 D5
13 D4
14 D3
To trim this circuit for full 12-bit precision, R2 and R3 need
to be adjustable over appropriate ranges. To trim, first have
the ADS7832 converting continually and apply +9.9927V
(+10V – 1.5LSB) at the input. Adjust R3 until the ADS7832
output toggles between the codes FFE hex and FFF hex.
This makes R3 extremely close to R1. Then, apply –9.9976V
(–10V + 0.5LSB) at the input, and adjust R2 until the
ADS7832 output toggles between 000 hex and 001 hex. At
each trim point, the current through the third resistor will be
almost zero, so that one trim iteration will be enough in most
cases. More iterations may be required if the op amp se-
lected has large offset voltage or bias currents, or if the +5V
reference is not precise.
D1 16
D2 15
10Ω
FIGURE 11. Power Supply and Reference Decoupling.
recommended in microprocessor applications to prevent
beat-frequency problems.
This circuit can also be used to adjust gain and offset errors
due to the components preceding the ADS7832, to match the
performance of the self-calibration provided by the con-
verter.
Note that the electrical specification tables are based on
using an external 2MHz clock. Typically, the specified
accuracy is maintained for clock frequencies between 0.5
and 2.4MHz.
®
14
ADS7832
BB [ BURR-BROWN CORPORATION ]
