LT1168 Datasheet, PDF (16/20 Pages) Linear Technology – Low Power, Single Resistor Gain Programmable, Micropower Precision Instrumentation Amplifier

LT1168
APPLICATIO S I FOR ATIO
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.
Low I_B Favors High Impedance Bridges, Lowers
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.
+IN
PATIENT
GND
–IN
PATIENT/CIRCUIT
PROTECTION/ISOLATION
C1
0.01µF
R1
12k
R2
1M
2
1
1/2
LT1112
3
3V
3
8
+
7
R3
30k
R_G
6k
R4
30k
1
LT1168
6
G = 10
5
2_–
4
–3V
A_V = 101
POLE AT 1kHz
0.3Hz
HIGHPASS
C2
3V
0.47µF
5₊ 8
R6
1/2
7
1M
LT1112
6_–
4
R7
–3V 10k
R8
100Ω
C3
15nF
OUTPUT
1V/mV
1168 F08
Figure 8. Nerve Impulse Amplifier
15V
3+
7
8
PRECISION
THERMISTOR
R_T
LT1634-1.25
1
2–
LT1168
REF
5
4
–15V
22k
1168 F09
6
V_OUT
=
1.25
•
49.4kΩ
R_T
–15V
Figure 9. Precision Temperature Without Precision Resistors
14
THERMOMETRICS
THERMO
DC95F103W
METRICS
12
DC95G104Z
10
8
6
4 YSI #44006
2
YSI #44011
0
–40 –20
0 20 40 60 80 100 120
TEMPERATURE (°C)
1168 F10
Figure 10. Response of Figure 9 for Various Thermistors
16