LT1167
17
1167fc
APPLICATIONS INFORMATION
–
+
3
+IN
R
X
V
X
I
L
–IN
8
1
1167 F07
–V
S
V
S
5
2
3
4
7
6
1/2
LT1464
R
G
2
1
LOAD
I
L
= =
[(+IN) – (–IN)]G
R
X
V
X
R
X
G = + 1
49.4kΩ
R
G
–
+
LT1167
REF
Figure 7. Precision Voltage-to-Current Converter
2
2
–IN
PATIENT
GROUND
OUTPUT
1V/mV
+IN
1
1
8
R6
1M
R7
10k
R8
100Ω
1167 F08
A
V
= 101
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
0.01μF
R
G
6k
3
3
R2
1M
C2
0.47μF
0.3Hz
HIGHPASS
C3
15nF
PATIENT/CIRCUIT
PROTECTION/ISOLATION
–
+
LT1167
G = 10
+
–
Figure 8. Nerve Impulse Amplifier
has the effect of improving the resolution of the current
source to 3pA, which is the maximum I
B
of the LT1464A.
Replacing R
G
with a programmable resistor greatly
increases the range of available output currents.
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
G
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
attenuated. Since the DC portion of the signal is not
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
B
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 dis-
sipation 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.
