Datasheet AD654 (Analog Devices) - 6

AD654 (OPTIONAL)
linearity, it is unnecessary for the end-user to perform this tedious
C AD654
and time consuming test on a routine basis. Sufficient FS calibration trim range must be provided to accom-
RCOMP
modate the worst-case sum of all major scaling errors. This
R1 R2
includes the AD654’s 10% full-scale error, the tolerance of the
VIN
fixed scaling resistor, and the tolerance of the timing capacitor. Therefore, with a resistor tolerance of 1% and a capacitor tolerance Figure 3b. Bias Current Compensation—Negative Inputs of 5%, the fixed part of the scaling resistor should be a maximum If the AD654’s 1 mV offset voltage must be trimmed, the trim of 84% of nominal, with the variable portion selected to allow must be performed external to the device. Figure 3c shows an 116% of the nominal. optional connection for positive inputs in which ROFF1 and If the input is in the form of a negative current source, the scaling ROFF2 add a variable resistance in series with RT. A variable resistor is no longer required, eliminating the capability of trim- source of ± 0.6 V applied to ROFF1 then adjusts the offset ± 1 mV. ming FS frequency in this fashion. Since it is usually not practical Similarly, a ± 0.6 V variable source is applied to ROFF in Fig- to smoothly vary the capacitance for trimming purposes, an ure 3d to trim offset for negative inputs. The ± 0.6 V bipolar alternative scheme such as the one shown in Figure 4 is needed. source could simply be an AD589 reference connected as shown Designed for a FS of 1 mA, this circuit divides the input into two in Figure 3e.
R2 100
V
AD654 AD654 10k
V
R4 VIN 392
V
R3 I 1k S
V
ROFF2 20
V
5k
V
8.25k
V
R1 100
V
ROFF1 I 1mA R 10k
V
FS ROFF I * S 100k
V
f =
6
0.6V (20V) CT –V
6
0.6V
Figure 3c. Offset Trim Positive Input (10 V FS)
*OPTIONAL OFFSET TRIM
6
0.6V
Figure 4. Current Source FS Trim
ROFF
and flowing into Pin 3; it constitutes the signal current I
5.6M
V T to be
AD654
converted. The second path, through another 100 Ω resistor R2,
10k
V carries the same nominal current. Two equal valued resistors offer the best overall stability, and should be either 1% discrete
8.25k
V
5k
V film units, or a pair from a common array.
VIN
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 Figure 3d. Offset Trim Negative Input (–10 V FS) (IS) is divided by a factor of two in this network. To achieve the same conversion scale factor, C
R1
T must be reduced by a factor of
10k
V two. This results in a transfer unique to this hookup:
+5V R3
f = IS
+ 10k
V (20 V ) C
R5
6 T
0.6V AD589 100k
V
–
For calibration purposes, resistors R3 and R4 are added to the
R4 10k R2
V network, allowing a ± 15% trim of scale factor with the values
10k
V
–5V
shown. By varying R4’s value the trim range can be modified to accommodate wider tolerance components or perhaps the cali- Figure 3e. Offset Trim Bias Network bration tolerance on a current output transducer such as the AD592 temperature sensor. Although the values of R1–R4 shown
FULL-SCALE CALIBRATION
are valid for 1 mA FS signals only, they can be scaled upward Full-scale trim is the calibration of the circuit to produce the proportionately for lower FS currents. For instance, they should desired output frequency with a full-scale input applied. In most be increased by a factor of ten for a FS current of 100 µA. cases this is accomplished by adjusting the scaling resistor RT. Precise calibration of the AD654 requires the use of an accurate In addition to the offsets generated by the input amplifier’s bias voltage standard set to the desired FS value and an accurate and offset currents, an offset voltage induced parasitic current frequency meter. A scope is handy for monitoring output wave- arises from the current fork input network. These effects are shape. Verification of converter linearity requires the use of a minimized by using the bias current compensation resistor ROFF switchable voltage source or DAC having a linearity error below and offset trim scheme shown in Figure 3e. ±0.005%, and the use of long measurement intervals to mini- Although device warm-up drifts are small, it is good practice to mize count uncertainties. Since each AD654 is factory tested for allow the devices operating environment to stabilize before trim, REV. C –5–