TS1002/TS1004
response is observed as there will appear noticeable
peaking/ringing in the output transient response.
If any amplifier is used in an application that requires
driving larger capacitive loads, an isolation resistor
between the output and the capacitive load should
be used as illustrated in Figure 5.
Figure 4:
A Two Op Amp Instrumentation Amplifier.
The circuit utilizes the classic two op amp
instrumentation amplifier topology with four resistors
to set the gain. The equation is simply that of a
noninverting amplifier as shown in the figure. The
two resistors labeled R1 should be closely matched
to each other as well as both resistors labeled R2 to
ensure
acceptable
common-mode
rejection
performance.
Resistor networks ensure the closest matching as
well as matched drifts for good temperature stability.
Capacitor C1 is included to limit the bandwidth and,
therefore, the noise in sensitive applications. The
value of this capacitor should be adjusted depending
on the desired closed-loop bandwidth of the
instrumentation amplifier. The RC combination
creates a pole at a frequency equal to 1/(2 π ×
R1C1). If the AC-CMRR is critical, then a matched
capacitor to C1 should be included across the
second resistor labeled R1.
Because these amplifiers accept rail-to-rail inputs,
their input common mode range includes both
ground and the positive supply of 1.5V. Furthermore,
their rail-to-rail output range ensures the widest
signal range possible and maximizes the dynamic
range of the system. Also, with their low supply
current of 0.6μA per amplifier, this circuit consumes
a quiescent current of only ~1.3μA, yet it still exhibits
a 1-kHz bandwidth at a circuit gain of 2.
Driving Capacitive Loads
While the amplifiers’ internal gain-bandwidth product
is 4kHz, both are capable of driving capacitive loads
up to 50pF in voltage follower configurations without
any additional components. In many applications,
however, an operational amplifier is required to drive
much larger capacitive loads. The amplifier’s output
impedance and a large capacitive load create
additional phase lag that further reduces the
amplifier’s phase margin. If enough phase delay is
introduced, the amplifier’s phase margin is reduced.
The effect is quite evident when the transient
Figure 5:
Using an External Resistor to Isolate a C
LOAD
from
the Amplifer’s Output.
Table 1 illustrates a range of R
ISO
values as a
function of the external C
LOAD
on the output of these
amplifiers. The power supply voltage applied on the
these amplifiers at which these resistor values were
determined empirically was 1.8V. The oscilloscope
capture shown in Figure 6 illustrates a typical
transient response obtained with a C
LOAD
= 500pF
and an R
ISO
= 50kΩ. Note that as C
LOAD
is increased
a smaller R
ISO
is needed for optimal transient
response.
External Capacitive
Load, C
LOAD
0-50pF
100pF
500pF
1nF
5nF
10nF
External Output
Isolation Resistor, R
ISO
Not Required
120kΩ
50kΩ
33kΩ
18kΩ
13kΩ
TS1002_4DS r1p0
Page 9
RTFDS
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