TS1005
and an RISO = 120kΩ. Note that as CLOAD is
increased a smaller RISO is needed for optimal
transient response.
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.
Figure 5: Using an External Resistor to Isolate a CLOAD from
the TS1005’s Output
External Capacitive
Load, CLOAD
External Output
Isolation Resistor, RISO
Because the TS1005 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
1.3μA, this circuit consumes a quiescent current of
only ~2.7μA, yet it still exhibits a 2-kHz bandwidth at
a circuit gain of 2.
0-50pF
100pF
500pF
1nF
5nF
10nF
Not Required
120kΩ
50kΩ
33kΩ
18kΩ
13kΩ
In the event that an external RLOAD in parallel with
CLOAD 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
Driving Capacitive Loads
While the TS1005’s internal gain-bandwidth product
is 20kHz, 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.
load resistor RLOAD
.
VIN
VOUT
If the TS1005 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.
values as a
Table 1 illustrates a range of RISO
function of the external CLOAD on the output of the
TS1005. The power supply voltage used on the
TS1005 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 CLOAD = 100pF
TS1005DS r1p0
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RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
