16

LT1168

a ground for the common mode signal. C1 was chosen to

maintain the stability of the patient ground. The LT1168’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 impor-

tant, 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 350µA of

supply current makes the LT1168 ideal for battery-pow-

ered applications. Total supply current for this application

is 1.05mA. Proper safeguards, such as isolation, must be

added to this circuit to protect the patient from possible

harm.

Dissipation

The LT1168’s low supply current, low supply voltage

operation and low input bias currents allow it to fit nicely

into battery-powered applications. Low overall power

dissipation necessitates using higher impedance bridges.

The single supply pressure monitor application on the

front of this data sheet, shows the LT1168 connected to

the differential output of a 3.5k bridge. The picoampere

input bias currents keep the error caused by offset current

to a negligible level. The LT1112 level shifts the LT1168’s

reference pin and the ADC’s analog ground pins above

ground. The LT1168’s and LT1112’s combined power

dissipation is still less than the bridge’s. This circuit’s total

supply current is just 2.2mA.

Figure 8. Nerve Impulse Amplifier

APPLICATIO S I FOR ATIO

2

2

–IN

PATIENT

GND

OUTPUT

1V/mV

+IN

1

1

8

R6

1M

R7

10k

R8

100

Ω

1168 F08

POLE AT 1kHz

5

5

4

–3V

–3V

3V

3V

7

6

8

4

7

6

1/2

LT1112

1/2

LT1112

R4

30k

R3

30k

R1

12k

C1

0.01

µF

6k

3

3

R2

1M

C2

0.47

µF

0.3Hz

HIGHPASS

C3

15nF

PATIENT/CIRCUIT

PROTECTION/ISOLATION

–

+

LT1168

G = 10

+

–

Figure 9. Precision Temperature Without Precision Resistors

1168 F09

15V

–15V

–15V

6

49.4k

Ω

7

3

8

1

2

4

22k

LT1634-1.25

PRECISION

THERMISTOR

5

LT1168

REF

Figure 10. Response of Figure 9 for Various Thermistors

TEMPERATURE (

°C)

–40

8

10

12

80

100

1168 F10

6

4

0

–20

20

40

60

120

2

0

14

YSI #44006

YSI #44011

THERMOMETRICS

DC95F103W

THERMO

METRICS

DC95G104Z