Datasheet

~
[
AD521
R.
~
":'
a). Transformer Coupled, Direct Return
R.
~
b). Thermocouple, Direct Return
R.
c). AC Coupled, Indirect Return
Figure3. Ground Rerums for "Floating" Transducers
7
VOUT
1. ~~~~~:~~:~CK UPGAINLOSTBYR
2. INPUT SIGNAL MUST BE REDUCED IN
PROPORTION TO POWER SUPPLY VOLTAGE LEVEL
Figure 4. Operating Conditions for V/~VS= 10V
V+
+
GAIN.~
Villi
RO
V-
1
Cx = 10fJ1rftwhen ft is the desired bandwidth.
(ft in kHz, Cx in J.l.F)
Figure5. Optional Compensation Circuit
R~lL--l1
INPUT OFFSET AND OUTPUT OFFSET
When specifying offsets and other errors in an operational
amplifier, it is often convenient to refer these errors to the
inputs. This enables the user to calculate the maximum error
he would see at the output with any gain or circuit configura-
tion. An op amp with 1mV of input offset voltage, for
example, would produce 1V of offset at the output in a gain
of 1000 configuration.
In the case of an instrumcntation amplifier, where the gain is
controlled in the amplifier, it is more convenient to separate
errors into two categories. Those errors which simply add to
the output signal and are unaffected by the gain can be classi-
fied as output errors. Those which act as if they are associated
with the input signal, such that their effect at the output is
proportional to the gain, can be classified as input errors.
As an illustration, a typical ADS21 might have a +30mV output
offset and a -o.7mV input offset. In a unity gain configuration,
a
the total output offset would be +29.3mV or the sum of the ~
tWo. At a gain of 100, the output offset would be -40mV or:
30mV + 100(-o.7mV) =-40mV.
By separating these errors, one can evaluate the total error
independent of the gain settings used, similar to the situation
with the input offset specifications on an op amp. In a given
gain configuration, both errors can be combined to give a total
error referred to the input (R.T.I.) or output (R.T.O.) by the
following formula:
Total Error R.T.I. =input error + (output error/gain)
Total Error R.T.O. =(Gain x input error) + output error
The offset trim adjustment (pins 4 and 6, Figure 2) is associ-
ated primarily with the output offset. At any gain it can be
used to introduce an output offset equal and opposite to the
input offset voltage multiplied by the gain. As a result, the
total output offset can be reduced to zero.
As shown in Figure 6, the gain range on the ADS 21 can be
extended considerably by adding an attenuator in the sense
terminal feedback path (as well as adjusting the ratio, Rs/~).
Since the sense terminal is the inverting input to the output
amplifier, the additional gain to the output is controlled by
Rl and Rz. This gain factor is 1 + Rz/Rl'
RI
V,
OUTPUT COMMON
VOUT
V2
R2
R,
VOUT . [VREF +(~)(VI -V2)][R' ;,R2]
Figure6. Circuit for utilizing some of the unique featuresof the
AD521. Note that gain changesintroduced by changingR1 and
R2 willhave a minimum effect on output offsst if the offsst is
carefully nulled at the highest gainsetting.
INSTRUMENTA nON AMPLIFIERS 4-21
Not Recommended for new Designs