LT1167AI datasheet(16/20 Pages) LINER

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LT1167

APPLICATIONS INFORMATION

important, R6 and C2 make up a 0.3Hz highpass filter.

The AC signal at LT1112’s Pin 5 is amplified by a gain of

101 set by (R7/R8) +1. The parallel combination of C3

and R7 form a lowpass filter that decreases this gain at

frequencies above 1kHz. The ability to operate at

±3V on

0.9mA of supply current makes the LT1167 ideal for

battery-powered applications. Total supply current for

this application is 1.7mA. Proper safeguards, such as

isolation, must be added to this circuit to protect the

patient from possible harm.

Low IB Favors High Impedance Bridges,

Lowers Dissipation

The LT1167’s low supply current, low supply voltage

operation and low input bias currents optimize it for

battery-powered applications. Low overall power dissi-

pation necessitates using higher impedance bridges. The

single supply pressure monitor application (Figure 9)

shows the LT1167 connected to the differential output of

a 3.5k bridge. The bridge’s impedance is almost an order

of magnitude higher than that of the bridge used in the

error-budget table. The picoampere input bias currents

keep the error caused by offset current to a negligible

level. The LT1112 level shifts the LT1167’s reference pin

and the ADC’s analog ground pins above ground. The

LT1167’s and LT1112’s combined power dissipation is

still less than the bridge’s. This circuit’s total supply

current is just 2.8mA.

+IN

–IN

1167 F07

–VS

1/2

LT1464

LOAD

IL =

[(+IN) – (–IN)]G

G =

+ 1

49.4k

Ω

–

LT1167

REF

Figure 8. Nerve Impulse Amplifier

–IN

PATIENT

GROUND

OUTPUT

1V/mV

+IN

10k

100

Ω

1167 F08

AV = 101

POLE AT 1kHz

–3V

1/2

LT1112

1/2

LT1112

30k

12k

0.01

µF

0.47

µF

0.3Hz

HIGHPASS

15nF

PATIENT/CIRCUIT

PROTECTION/ISOLATION

–

LT1167

G = 10

–

Nerve Impulse Amplifier

The LT1167’s low current noise makes it ideal for high

source impedance EMG monitors. Demonstrating the

LT1167’s ability to amplify low level signals, the circuit in

Figure 8 takes advantage of the amplifier’s high gain and

low noise operation. This circuit amplifies the low level

nerve impulse signals received from a patient at Pins 2

and 3. RG and the parallel combination of R3 and R4 set

a gain of ten. The potential on LT1112’s Pin 1 creates a

ground for the common mode signal. C1 was chosen to

maintain the stability of the patient ground. The LT1167’s

high CMRR ensures that the desired differential signal is

amplified and unwanted common mode signals are at-

tenuated. Since the DC portion of the signal is not

Figure 7. Precision Voltage-to-Current Converter

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