TS1002 datasheet(9/13 Pages) TOUCHSTONE

9 / 13 page

TS1002/TS1004

TS1002_4DS r1p0

Page 9

RTFDS

The

circuit

utilizes

the

classic

two

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

the

desired

closed-loop

bandwidth

the

instrumentation

amplifier.

The

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 0.6

μA per amplifier, this circuit consumes

a quiescent current of only ~1.3

μA, 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

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.

Table 1 illustrates a range of RISO values as a

function of the external CLOAD 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 CLOAD = 500pF

and an RISO = 50kΩ. Note that as CLOAD is increased

a smaller RISO is needed for optimal transient

response.

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 Amplifer’s Output.

Figure 4: A Two Op Amp Instrumentation Amplifier.

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