AD573
Figure 4a shows how the converter zero may be offset by up to
±3 bits to correct the device initial offset and/or input signal
offsets. As shown, the circuit gives approximately symmetrical
adjustment in unipolar mode.
20
19
18
17
16
15
14
13
12
11
LSB DB0
DB1
1
2
PIN 1
IDENTIFIER
HBE
LBE
DB2
DR
3
DIG COM
BIP OFF
DB3
4
DB4
5
AD573
TOP VIEW
(Not to Scale)
DB5
ANALOG COM
ANALOG IN
6
DB6
7
8
V–
DB7
DB8
CONVERT
9
10
V+
MSB DB9
Figure 2. AD573 Pin Connections
Full-Scale Calibration
The 5 kΩ thin-film input resistor is laser trimmed to produce a
current which matches the full-scale current of the internal
DAC—plus about 0.3%—when an analog input voltage of 9.990
volts (10 volts – 1 LSB) is applied at the input. The input resis-
tor is trimmed in this way so that if a fine trimming potentiom-
eter is inserted in series with the input signal, the input current
at the full-scale input voltage can be trimmed down to match
the DAC full-scale current as precisely as desired. However, for
many applications the nominal 9.99 volt full scale can be
achieved to sufficient accuracy by simply inserting a 15 Ω resis-
tor in series with the analog input to Pin 14. Typical full-scale
calibration error will then be within ±2 LSB or ±0.2%. If more
precise calibration is desired, a 50 Ω trimmer should be used
instead. Set the analog input at 9.990 volts, and set the trimmer
so that the output code is just at the transition between
11111111 10 and 11111111 11. Each LSB will then have a
weight of 9.766 mV. If a nominal full scale of 10.24 volts is de-
sired (which makes the LSB have a weight of exactly 10.00 mV),
a 100 Ω resistor and a 100 Ω trimmer (or a 200 Ω trimmer with
good resolution) should be used. Of course, larger full-scale
ranges can be arranged by using a larger input resistor, but lin-
earity and full-scale temperature coefficient may be compro-
mised if the external resistor becomes a sizeable percentage of
5 kΩ. Figure 3 illustrates the connections required for full-scale
calibration.
Figure 4a.
Figure 4. Offset Trims
Figure 4b.
Figure 5 shows the nominal transfer curve near zero for an
AD573 in unipolar mode. The code transitions are at the edges
of the nominal bit weights. In some applications it will be pref-
erable to offset the code transitions so that they fall between the
nominal bit weights, as shown in the offset characteristics.
Figure 5. AD573 Transfer Curve—Unipolar Operation
(Approximate Bit Weights Shown for Illustration, Nominal
Bit Weights ~ 9.766 mV)
This offset can easily be accomplished as shown in Figure 4b. At
balance (after a conversion) approximately 2 mA flows into the
Analog Common terminal. A 2.7 Ω resistor in series with this
terminal will result in approximately the desired 1/2 bit offset of
the transfer characteristics. The nominal 2 mA Analog Common
current is not closely controlled in manufacture. If high accu-
racy is required, a 5 Ω potentiometer (connected as a rheostat)
can be used as R1. Additional negative offset range may be ob-
tained by using larger values of R1. Of course, if the zero transi-
tion point is changed, the full-scale transition point will also
move. Thus, if an offset of 1/2 LSB is introduced, full-scale
trimming as described on the previous page should be done with
an analog input of 9.985 volts.
NOTE: During a conversion, transient currents from the Analog
Common terminal will disturb the offset voltage. Capacitive
decoupling should not be used around the offset network. These
transients will settle appropriately during a conversion. Capaci-
tive decoupling will “pump up” and fail to settle resulting in
conversion errors. Power supply decoupling, which returns to
analog signal common, should go to the signal input side of the
resistive offset network.
Figure 3. Standard AD573 Connections
Unipolar Offset Calibration
Since the Unipolar Offset is less than ±1 LSB for all versions of
the AD573, most applications will not require trimming. Figure
4 illustrates two trimming methods which can be used if greater
accuracy is necessary.
REV. B
–4–
ADI [ ADI ]
