Texas Instruments | Using the ADS7800 12 Bit ADC with Unipolar Input Signals | Application notes | Texas Instruments Using the ADS7800 12 Bit ADC with Unipolar Input Signals Application notes

Texas Instruments Using the ADS7800 12 Bit ADC with Unipolar Input Signals Application notes
APPLICATION BULLETIN
®
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USING THE ADS7800 12 BIT ADC
WITH UNIPOLAR INPUT SIGNALS
By R. Mark Stitt and Dave Thomas (602) 746-7445
The ADS7800 12 bit Sampling analog-to-digital-converter
is designed to operate with bipolar inputs of ±5V or ±10V.
With the addition of an external amplifier, the ADS7800 can
be used for 10V or 20V unipolar inputs. Four unipolar input
options are shown in this Bulletin.
grounded. There are four unipolar input range connections
to the ADS7800 that will produce ±2V VO when V1 or V2 is
connected to an appropriate offsetting voltage instead of
ground. A summary of the four cases is shown in Table I.
INPUT RANGE
±5V
±10V
0 to +10V
0 to –10V
0 to +20V
0 to –20V
ADS7800 UNIPOLAR VOLTAGE INPUT RANGES
INPUT
0
0
0
0
to
to
to
to
SEE FIGURE
+10V
–10V
+20V
–20V
2
3
4
5
R3 (R2 V1 + R1 V2)
R1 R2 + R1 R3 + R2 R3
V 1 + 2 V2
5
Where:
V1 = voltage at pin 1 (V)
V2 = voltage at pin 2 (V)
VO = voltage into ADC cell (V)
V1
V2
2V Ref
Out
1
2
R2
2.5k Ω
±2V VO
to ADC
12 Bits
Out
R3
2.5k Ω
3
The +2.0V reference output from the ADS7800 can be
amplified to provide the ±5V or ±10V offsetting voltage
needed. The circuits in Figures 2 to 5 show how to connect
inverting or noninverting amplifiers for the various input
ranges.
If desired, the ADC zero (negative full-scale) can be adjusted
by fine trimming the offsetting voltage with the 1kΩ pot. To
adjust the ADC zero, apply a 1/2 LSB voltage to the input
of the ADC and adjust the gain adjust pot so the LSB output,
pin 17, toggles between 1 and 0.
ADS7800
R1
5k Ω
4
FIGURE 1. ADS7800 Input-Voltage Scaling Resistor Network.
The 1/2 LSB zero-adjust voltage can be derived from a
resistor divider connected to a voltage source (See Application Bulletin AB-003, 004, and 005 for ±10V references).
Remember to consider the input impedance of the ADC
when using a resistor divider. In the 10V range, the input
impedance is 2.5kΩ + (2.5kΩ || 5kΩ) = 4.17kΩ, in the 20V
range the input impedance is 5kΩ + (2.5kΩ || 2.5kΩ) =
6.25kΩ. Recommended values for a zero-adjust divider are:
The internal ADC gives a zero to full-scale digital output
with ±2V at VO (the internal ADC node voltage shown in
Figure 1). When used in the standard bipolar voltage input
mode, with V2= 0 (i.e. V2 connected to analog ground), ±10V
input at pin 1 of the ADS7800 produces ±2V output at VO.
Similarly, ±5V input at pin 2 produces ±2V VO with V1

1990 Burr-Brown Corporation
SBAA044
VO
±2V
±2V
±2V
±2V
±2V
±2V
Adjustment of the offsetting voltage is required because the
absolute accuracy of the 2.0V ADS7800 reference output
may vary by a few percent. Adjust the 1kΩ pot for an
accurate offsetting voltage (e.g. V 2 = 5.000V or V1 =
10.000V). Even though the reference output of the ADS7800
does not have absolute accuracy, the gain of the ADC is
scaled to its value. Scaling the internal reference to generate
the offsetting voltage preserves gain accuracy with temperature and supply variation so long as the R 4/R5 resistor ratio
tracks with temperature and a low drift op amp, such as the
OPA177, is used.
since R2 = R3 and R1 = 2 R2,
VO =
V2
INPUT
0 (Ground)
INPUT
INPUT
–5.000V
+5.000V
TABLE I. ADS7800 Input Ranges (see Figure 1).
To understand how the circuits work, consider the ADS7800
input voltage divider network shown in Figure 1. Since the
input resistor divider network drives a high impedance at
VO, the transfer function is:
VO =
V1
0 (Ground)
INPUT
-10.000V
+10.000V
INPUT
INPUT
AB-019A
INPUT RANGE
(V)
0
0
0
0
to
to
to
to
+10
–10
+20
–20
1/2 LSB
(V)
VREF
(V)
RD1
(kΩ)
RD2
(Ω)
0.012
–0.012
0.024
–0.024
+10.00
–10.00
+10.00
–10.00
8.25
8.25
4.12
4.12
10.0
10.0
10.0
10.0
Printed in U.S.A. February, 1991
ADS7800
ADS7800
0 to 10V
In (V2 )
1kΩ
2
R2
2.5k Ω
±2V VO
to ADC
12 Bits
Out
0 to –10V
In (V2 )
R3
2.5k Ω
3
–10V
R5
49.9k Ω
1
R1
5k Ω
R4
9.53k Ω
1
R1
5k Ω
2
R2
2.5k Ω
±2V VO
to ADC
R3
2.5k Ω
3
+10V
4
12 Bits
Out
OPA177
4
47pF
47pF
R4
9.53k Ω
R5
39.2k Ω
OPA177
1kΩ
Nominal Value of R5/R4 = 50/10.
Nominal Value of R5/R4 = 40/10.
FIGURE 2. 12-Bit ADC with 0 to 10V Unipolar Input Range
Using the ADC7800.
1
0 to 20V
In (V1 )
2
1kΩ
–5V
R5
24.3k Ω
ADS7800
R1
5k Ω
ADS7800
±2V VO
to ADC
12 Bits
Out
0 to –20V
In (V1 )
R2
2.5k Ω
R3
2.5k Ω
3
R4
9.53k Ω
FIGURE 3. 12-Bit ADC with 0 to –10V Unipolar Input Range
Using the ADC7800.
1
R1
5k Ω
2
R2
2.5k Ω
±2V VO
to ADC
R3
2.5k Ω
3
+5V
12 Bits
Out
OPA177
4
4
47pF
47pF
R4
9.53k Ω
R5
14.7k Ω
OPA177
1kΩ
Nominal Value of R5/R4 = 25/10.
Nominal Value of R5/R4 = 15/10.
FIGURE 4. 12-Bit ADC with 0 to 20V Unipolar Input Range
Using the ADC7800.
FIGURE 5. 12-Bit ADC with 0 to –20V Unipolar Input Range
Using the ADC7800.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
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any BURR-BROWN product for use in life support devices and/or systems.
2
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