TS1003
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/(2π×R1C1).
If the AC-CMRR is critical, then a matched capacitor
to C1 should be included across the second resistor
labeled R1.
Because the TS1003 accepts rail-to-rail inputs, the
input common mode range includes both ground
and the positive supply of 1.5V. Furthermore, the
rail-to-rail output range ensures the widest signal
range possible and maximizes the dynamic range of
the system. Also, with its low supply current of
0.6μA,
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 TS1003’s internal gain-bandwidth product
is 4kHz, it is 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
response is observed as there will appear noticeable
peaking/ringing in the output transient response.
If the TS1003 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.
Table 1 illustrates a range of R
ISO
values as a
function of the external C
LOAD
on the output of the
TS1003. The power supply voltage used on the
TS1003 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
= 100pF
TS1003DS r1p0
Page 9
and an R
ISO
= 120kΩ.
Note that as C
LOAD
is
increased a smaller R
ISO
is needed for optimal
transient response.
Figure 5:
Using an External Resistor to Isolate a C
LOAD
from
the TS1003’s Output
In the event that an external R
LOAD
in parallel with
External Capacitive
External Output
Load, C
LOAD
Isolation Resistor, R
ISO
0-50pF
Not Required
100pF
120kΩ
500pF
50kΩ
1nF
33kΩ
5nF
18kΩ
10nF
13kΩ
C
LOAD
appears in the application, the use of an R
ISO
results in gain accuracy loss because the external
series R
ISO
forms a voltage-divider with the external
load resistor R
LOAD
.
V
IN
V
OUT
RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
