Datasheet

LTC6240/LTC6241/LTC6242
23
624012fe
Figure 9. Ultralow Noise Chopper Amplifi er
to achieve an extraordinarily low noise and low DC drift.
The performance of this circuit is suited for the demand-
ing transducer signal conditioning situations such as high
resolution scales and magnetic search coils.
The LTC1799’s output is divided down to form a 2-phase
925Hz square wave clock. This frequency, harmonically
unrelated to 60Hz, provides excellent immunity to harmonic
beating or mixing effects which could cause instabilities.
S1 and S2 receive complementary drive, causing A1 to
see a chopped version of the input voltage. A1’s square
wave output is synchronously demodulated by S3 and
S4. Because these switches are synchronously driven
with the input chopper, proper amplitude and polarity
information is presented to A2, the DC output amplifi er.
This stage integrates the square wave into a DC voltage,
providing the output. The output is divided down (R2 and
R1) and fed back to the input chopper where it serves as
a zero signal reference. Gain, in this case 1000, is set by
the R1-R2 ratio. Because A1 is AC coupled, its DC offset
and drift do not affect the overall circuit offset, resulting
in the extremely low offset and drift noted. The JFETs
have an input RC damper that minimizes offset voltage
contribution due to parasitic switch behavior, resulting in
the 1µV offset specifi cation.
APPLICATIONS INFORMATION
+
–
+
–
BIAS
10M
1µF
14
15
16
3
2
1
S4
S3
240k
1µF
OUTPUT
A2
LTC6241HV
A1
LTC6241HV
1µF
INPUT
10k
8
7
11
S1
S2
9
6
10
R2
10k
R1
10Ω
NOISE
OFFSET
DRIFT
OPEN-LOOP GAIN
I
= 40nV
P-P
0.1Hz TO 10Hz
= 1µV
= 0.05µV/°C
R2
10
= 10
= 500pA
+1
GAIN
=
9
0.01µF
Ø1
Ø1
Ø2
Ø2
6241 F09
= 0.1% METAL FILM RESISTOR
= 1% METAL FILM RESISTOR
*
**
= LTC201 QUAD
= LSK389
= LINEAR INTEGRATED SYSTEMS
FREMONT, CA
1µF
DIV
R
SET
LTC1799
V
+
74C90 ÷ 10
18.5kHz
OUT
74C74 ÷ 2
TO
Ø1
POINTS
TO
Ø2
POINTS
Q Q
54.2k*
TO LTC201 V
+
PIN
5V
5V
–5V
5V 5V
925Hz
TO LTC201 V
–
PIN
1µF
898Ω**
5V
–5V
898Ω**
LSK389
30.1Ω
499Ω**
+
+