TS1005 datasheet(9/11 Pages) TOUCHSTONE

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TS1005

TS1005DS r1p0

Page 9

RTFDS

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 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.

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.

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.

Table 1 illustrates a range of RISO values as a

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

and an RISO = 120kΩ. Note that as CLOAD is

increased a smaller RISO is needed for optimal

transient response.

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

load resistor RLOAD.

External Capacitive

Load, CLOAD

External Output

Isolation Resistor, RISO

0-50pF

Not Required

100pF

120kΩ

500pF

50kΩ

1nF

33kΩ

5nF

18kΩ

10nF

13kΩ

Figure 5: Using an External Resistor to Isolate a CLOAD from

the TS1005’s Output

VIN

VOUT

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