AD654
(OPTIONAL)
C
AD654
R
COMP
V
IN
R1
R2
Figure 3b. Bias Current Compensation—Negative Inputs
If the AD654’s 1 mV offset voltage must be trimmed, the trim
must be performed external to the device. Figure 3c shows an
optional connection for positive inputs in which R
OFF1
and
R
OFF2
add a variable resistance in series with R
T
. A variable
source of
±
0.6 V applied to R
OFF1
then adjusts the offset
±
1 mV.
Similarly, a
±
0.6 V variable source is applied to R
OFF
in Fig-
ure 3d to trim offset for negative inputs. The
±
0.6 V bipolar
source could simply be an AD589 reference connected as shown
in Figure 3e.
AD654
V
IN
R
OFF2
5k
20
R
OFF1
10k
0.6V
10k
linearity, it is unnecessary for the end-user to perform this tedious
and time consuming test on a routine basis.
Sufficient FS calibration trim range must be provided to accom-
modate the worst-case sum of all major scaling errors. This
includes the AD654’s 10% full-scale error, the tolerance of the
fixed scaling resistor, and the tolerance of the timing capacitor.
Therefore, with a resistor tolerance of 1% and a capacitor tolerance
of 5%, the fixed part of the scaling resistor should be a maximum
of 84% of nominal, with the variable portion selected to allow
116% of the nominal.
If the input is in the form of a negative current source, the scaling
resistor is no longer required, eliminating the capability of trim-
ming FS frequency in this fashion. Since it is usually not practical
to smoothly vary the capacitance for trimming purposes, an
alternative scheme such as the one shown in Figure 4 is needed.
Designed for a FS of 1 mA, this circuit divides the input into two
R2
100
R4
392
I
S
AD654
R3
1k
8.25k
R1
100
1mA
FS
I
R
*
R
OFF
100k
f=
I
S
(20V) C
T
–V
0.6V
*OPTIONAL
OFFSET TRIM
Figure 3c. Offset Trim Positive Input (10 V FS)
0.6V
R
OFF
5.6M
10k
Figure 4. Current Source FS Trim
AD654
V
IN
8.25k
5k
and flowing into Pin 3; it constitutes the signal current I
T
to be
converted. The second path, through another 100
Ω
resistor R2,
carries the same nominal current. Two equal valued resistors
offer the best overall stability, and should be either 1% discrete
film units, or a pair from a common array.
Since the 1 mA FS input current is divided into two 500
µA
legs
(one to ground and one to Pin 3), the total input signal current
(I
S
) is divided by a factor of two in this network. To achieve the
same conversion scale factor, C
T
must be reduced by a factor of
two. This results in a transfer unique to this hookup:
f
=
I
S
(20
V
)
C
T
Figure 3d. Offset Trim Negative Input (–10 V FS)
R1
10k
+5V
R3
10k
+
AD589
–
R2
10k
R5
100k
0.6V
R4
10k
–5V
Figure 3e. Offset Trim Bias Network
FULL-SCALE CALIBRATION
Full-scale trim is the calibration of the circuit to produce the
desired output frequency with a full-scale input applied. In most
cases this is accomplished by adjusting the scaling resistor R
T
.
Precise calibration of the AD654 requires the use of an accurate
voltage standard set to the desired FS value and an accurate
frequency meter. A scope is handy for monitoring output wave-
shape. Verification of converter linearity requires the use of a
switchable voltage source or DAC having a linearity error below
±
0.005%, and the use of long measurement intervals to mini-
mize count uncertainties. Since each AD654 is factory tested for
REV. B
–5–
For calibration purposes, resistors R3 and R4 are added to the
network, allowing a
±
15% trim of scale factor with the values
shown. By varying R4’s value the trim range can be modified to
accommodate wider tolerance components or perhaps the cali-
bration tolerance on a current output transducer such as the
AD592 temperature sensor. Although the values of R1–R4 shown
are valid for 1 mA FS signals only, they can be scaled upward
proportionately for lower FS currents. For instance, they should
be increased by a factor of ten for a FS current of 100
µA.
In addition to the offsets generated by the input amplifier’s bias
and offset currents, an offset voltage induced parasitic current
arises from the current fork input network. These effects are
minimized by using the bias current compensation resistor R
OFF
and offset trim scheme shown in Figure 3e.
Although device warm-up drifts are small, it is good practice to
allow the devices operating environment to stabilize before trim,
AD [ ANALOG DEVICES ]
