Chopper Amplifiers Complement a DC
Accurate Low-Pass Filter – Design Note 9
Nello Sevastopoulos
Monolithic switched-capacitor low-pass filters, although
they offer precise frequency responses, cannot usually be used for DC accurate applications because of
their prohibitive DC offsets and poor gain linearity. The
LTC ®1062, however, is quite different from currently
available low-pass switched capacitor filters because it
uses an external (R, C) to isolate the IC from the inputsignal DC path and to provide antialiasing for incoming
signals larger than half its clock frequency. The LTC1062
is ideal when used in conjunction with high performance
chopper-stabilized op amps.
The LTC1050 is an ultra low offset, low noise chopper with
the sampling capacitors internal. It can remove residual
clock noise without adding further DC error. Also, the
internal capacitor minimizes board area.
Figure 1 shows a low cost, 7th order DC accurate, 10Hz
low-pass filter where amplitude and phase response
closely approximate a Bessel filter. The required clock
frequency is 2kHz, thus yielding clock to cutoff frequency
ratio of 200:1.
VIN
R'
196k
R
16k
C
0.47µF
1
–8V
1N4148
0.1µF
2
3
4
The LTC1050 is configured as unity gain 2nd order lowpass filter which center frequency is (1.2πR'C') = 1.72 •
f CUTOFF = 17.2Hz and Q = 0.5. Figure 2 shows the amplitude
response of the filter, and Figure 3 shows a well behaved
transient response for which Bessel filters are famous. The
power supplies used were ±8V to provide a total DC input
common-mode range of ±6V. The measured wideband
noise was 52µVRMS. The clock, and R, C values of Figure 1
can be easily modified to provide a 7th order Butterworth
10Hz filter, such as: fCLK = 1kHz, R = 26.7k, C = 1µF, R' =
165k, C1 = 0.2µF and C2 = 0.047µF. The diode at LTC1062
pin 3 should be used to protect the device from incoming
signals above the power supplies.
In Figure 4, an external (R1, R2, C1) network is used at the
input-output of the internal buffer of the LTC1062 (pins
7 and 8), to provide an additional 2 pole, Q = 0.707, highpass filter. The filter output at pin 8 is bandpass, Figure 5,
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C
0.047µF
8
LTC1062
3
C1 = C2 = C'
2
+
7
6
LTC1050
–
4
–8V
7
DN009 F01
6
5
8V
R'
196k
fCLK = 2kHz
8V
0.1µF
Figure 1. Combining the LTC1050 Chopper Op Amp with the LTC1062 to
Provide a 10Hz, DC Accurate Lowpass Bessel Filter
04/88/9_conv
VOUT
–3dB
0
GAIN (dB)
–15
–30
–45
–60
–75
–90
–105
10
AMPLITUDE RESPONSE (Hz)
1
100
138/338 G09
Figure 2. Amplitude Response of Figure 1, Providing a
Close Approximation to a Bessel Filter
whereas the DC accurate Butterworth low-pass filter is
still available at the output of the LTC1050. This circuit
allows the user to separate the DC and AC components
of an incoming signal, VIN. Here, the LTC1050 buffers the
low-pass filter section of the overall bandpass filter. For a
Q = 0.707, the design equation for the high-pass sections
are straightforward: set R2 = 2R1; and then the high-pass
Figure 3. Transient Response of the Bessel Low-Pass
Filter of Figure 1
cutoff frequency is, fC = (0.707/2πR1C1). The circuit in
Figure 4 can be easily operated with a single supply,
because resistor R2 and capacitor(s) C1 of the high-pass
section, also DC bias pin 7 at mid supplies independently
of the DC input voltage. If only a single supply is available,
simply bias the bottom side of R2 at half supply.
VOUT2
(BANDPASS)
VIN
C
0.01µF
1
2
3
C1
0.022µF
R1
100k
8
LTC1062
4
V– = –5V
0.1µF
7
2
+
3
–
5V
7
LTC1050
VOUT1
(LOWPASS)
–20
4
C1
0.022µF R2
200k
–30
–40
–50
–60
–70
6
5
–10
6
–5V
–3dB
0
fINGAIN (dB)
R
20.5k
145kHz CLOCK
V+ = 5V
0.1µF
–80
10
100
1k
fIN, FREQUENCY RESPONSE (Hz)
DN009 F05
DN009 F04
Figure 4. A Bandpass, DC Accurate Lowpass Filter
Combination used to Extract the AC Information from a
DC + AC input Signal
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www.linear.com/LTC1062
Linear Technology Corporation
10k
Figure 5. Frequency Response of the Bandpass
Filter Output, VOUT2, of Figure 4
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