LT1167 Linear Technology, LT1167 Datasheet - Page 16

• Manufacturer Part Number: LT1167
• Description: Single Resistor Gain Programmable/ Precision Instrumentation Amplifier
• Manufacturer: Linear Technology

APPLICATIONS
LT1167
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. R
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
16
+IN
–IN
GROUND
PATIENT
Figure 7. Precision Voltage-to-Current Converter
G
I
G =
L
and the parallel combination of R3 and R4 set
–IN
+IN
=
R
R
49.4k
V
G
X
X
3
8
1
2
R
=
G
+
–
[(+IN) – (–IN)]G
+ 1
PATIENT/CIRCUIT
PROTECTION/ISOLATION
U
LT1167
–V
R
X
1
0.01 F
S
4
C1
V
S
INFORMATION
7
U
REF
1M
R2
LT1112
5
1/2
1
12k
R1
–
+
3
2
6
LT1464
W
1/2
–
+
R3
30k
R4
30k
R
V
X
X
2
3
A
POLE AT 1kHz
V
Figure 8. Nerve Impulse Amplifier
= 101
R
6k
LOAD
G
U
1
3
8
2
I
+
–
L
1167 F07
LT1167
G = 10
–3V
3V
4
7
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 I
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.
5
B
6
Favors High Impedance Bridges,
0.47 F
C2
0.3Hz
HIGHPASS
R6
1M
100
R8
5
6
+
–
LT1112
1/2
–3V
3V
15nF
C3
4
8
10k
R7
7
OUTPUT
1V/mV
1167 F08