TS1001
transient response obtained with a CLOAD = 500pF
and an RISO = 50kꢀ. Note that as CLOAD is increased
a smaller RISO is needed for optimal transient
response.
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.
Figure 5: Using an External Resistor to Isolate a CLOAD from
the TS1001’s Output
External Capacitive
Load, CLOAD
0-50pF
External Output
Isolation Resistor, RISO
Because the TS1001 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.
Not Required
120kꢀ
50kꢀ
100pF
500pF
1nF
33kꢀ
5nF
18kꢀ
10nF
13kꢀ
In the event that an external RLOAD in parallel with
LOAD appears in the application, the use of an RISO
results in gain accuracy loss because the external
series RISO forms a voltage-divider with the external
C
Driving Capacitive Loads
While the TS1001’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
load resistor RLOAD
.
VIN
VOUT
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 TS1001 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 6: TS1001 Transient Response for RISO = 50kꢀ and
CLOAD = 500pF
.
Table 1 illustrates a range of RISO values as a
function of the external CLOAD on the output of the
TS1001. The power supply voltage used on the
TS1001 at which these resistor values were
determined empirically was 1.8V. The oscilloscope
capture shown in Figure 6 illustrates a typical
TS1001DS r1p0
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RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
