QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 626A
DC ACCURATE, TUNABLE, LOW PASS FILTER
DC626A-A, LTC1569-6
DC626A-B, LTC1569-7
DESCRIPTION
The LTC1569-6 and LTC1569-7 are monolithic 10th
order linear phase low pass filters. These are the first
sampled data filters in the industry to provide accurate
control of the cutoff frequency without the need for an
external clock. Offered in the 8-lead SOIC package, the
LTC1569 family is tunable with a single resistor. The
LTC1569’s ease of use, accurate tuning and small
package combined with its precision performance and
high selectivity is practically impossible to achieve with
discrete components. Its root raised cosine gain re-
sponse and linear phase results in excellent lowpass
characteristics with steep selectivity ideal for use in
data communication and data acquisition systems
LTC is a trademark of Linear Technology Corporation
The filter attenuation is 50dB at 1.5 times the filter cut-
off and 80dB at 6 times the cutoff frequency. Thus, the
LTC1569 family is an excellent choice in applications
with demanding specifications for rejection of frequency
components outside the passband. With up to 300kHz
cutoff, the LTC1569-7 is intended for systems needing
maximum bandwidth whereas the LTC1569-6 offers a
low power solution with consumption as low as 8mW
(fc = 4kHz). Operating from 3V to ±5V supplies, the
LTC1569-6 and LTC1569-7 are available in the 8-pin
SOIC package.
Design files for this circuit board are available.
Call the LTC factory.
Figure 1. Demo Circuit Board 626A-A
Note:
U1 on Demo Circuit 626A-A is an LTC1569-6.
Demo Circuit 626A-B is the same as 626A-A except for an LTC1569-7 installed as U1.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 626A
DC ACCURATE, TUNABLE, LOW PASS FILTER
Table 1. Key LTC1569 Family Electrical Characteristics
PARAMETER
CONDITION
MIN
TYP
MAX
Output Voltage Range
(Note1)
VS = 3V, AGND Voltage (Pin 3) = 1.11V
VS = 5V, AGND Voltage (Pin 3) = 2.0V
(The output AC voltage swings
around a DC voltage equal to the VS = ±5V, Pin 5 Shorted to Pin 7
voltage on pin 3)
VS = 3V
VS = 5V
VS = ±5V
REXT = 10K, Pin 5
Shorted to Pin 7
VS = 3V
VS = 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
VS = 3V to 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
1.9V
p-p
2.1V
p-p
3.7V
p-p
3.9V
p-p
8.0V p-p
(fclk 7MHz LTC11659-7)
(fclk 4MHz LTC11659-6)
Output DC Offset
±15mV
25µV/ C (-25µV/C for LTC1569-7)
25µV/ C (-25µV/C for LTC1569-7)
75µV/C
4.1MHz
5MHz
5MHz
5MHz
7MHz
10MHz
1.5kHz
3.0kHz
Output DC Offset Drift
Max Clock Frequency LTC1569-6
Max Clock Frequency LTC1569-7
Minimum Clock Frequency
LTC1569-6 and LTC1569-7
Table 2. TYPICAL FCUTOFF VARIATION VS. REXT
LTC1569-6
LTC1569-7
REXT
10k
20.18k
40.2k
Typical
fCUTOFF
64kHz
32kHz
16kHz
Typical
fCUTOFF
Variation
±1%
±2.0%
±3.5%
REXT
10K
20.18K
40.2K
Typical
fCUTOFF
128kHz
64kHz
32kHZ
Typical
fCUTOFF
Variation
±1%
±2.0%
±3.5%
On DC 626A, REXT is equal to REXT1 + REXT2
(REXT1 is a 50k 10% potentiometer and REXT2 is a 3.4k 1% resistor).
Maximum REXT value 81k (the frequency error for REXT > 50k can be greater than ±3.5%).
Note 1:
In a single power supply circuit, the Output Voltage Range is maximized if the AGND voltage (on pin 3) is
[0.37x V+] (1.85V for Vs=5V and 1.11V for Vs=3V). The AGND voltage range is [0.37x V+] to [0.5x V+].
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 626A
DC ACCURATE, TUNABLE, LOW PASS FILTER
LTC1569-6 and LTC1569-7 Equations
(Calculating the value of the external resistance that sets the internal clock frequency)
FCUTOFF
is the -3dB Frequency
REXT
is the resistance value connected from pin 7 to pin 6
N
is the internal divider value,
N = 1, 4 or 16
LTC1569-6
FCUTOFF =
(64kHz / N)•(10k / REXT),
REXT in k
REXT =
(64kHz / FCUTOFF)•(10k / N), FCUTOFF in kHz
IF
IF 4
IF 16
(64kHz / FCUTOFF) < 4 THEN N = 1
(64kHz / FCUTOFF) < 16 THEN N = 4
(64kHz / FCUTOFF)
THEN N = 16
REXT in k
FCUTOFF in kHz
LTC1569-7
FCUTOFF = (128kHz
/ N)•(10k / REXT),
REXT =
(128kHz / FCUTOFF)•(10k / N),
IF
IF 4
IF 16
(128kHz / FCUTOFF) < 4 THEN N = 1
(128kHz / FCUTOFF) < 16 THEN N = 4
(128kHz / FCUTOFF)
THEN N = 16
Calculation procedure for setting the value of potentiometer REXT1
1. A.
For an LTC1569-6 (DC626A-A), calculate (64kHz / FCUTOFF)
B.
For an LTC1569-7 (DC626A-B), calculate (128kHz / FCUTOFF)
and select N
and select N
2. A.
For an LTC1569-6, calculate REXT = (64kHz / FCUTOFF)•(10k / N)
B.
For an LTC1569-7, calculate REXT = (128kHz / FCUTOFF)•(10k / N)
3.
Calculate the value of potentiometer REXT1, REXT1 = (REXT - 3.4k)
Example: Set REXT1 for FCUTOFF = 16kHz
STEP 1. A. N=4 for an LTC1569-6, (64kHz /16kHz)=4 and
B. N=4 for an LTC1569-7, (128kHz/16kHz)=8
STEP 2. A. REXT for LTC1569-6 = [640kHz/(16kHz •4)]k=10k and
B. REXT for LTC1569-7 = [1280kHz/(16kHz•4)]k=20k
STEP 3.
REXT1 for LTC1569-6 = [10k - 3.4k]= 6.6k and
REXT1 for LTC1569-7 = [20k - 3.4k]=16.6k
The Frequency Of The External Clock, FCLOCK (Pin 6 connected to V-)
LTC1569-6 FCLOCK = 64x FCUTOFF
LTC1569-7 FCLOCK = 32x FCUTOFF
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 626A
DC ACCURATE, TUNABLE, LOW PASS FILTER
Figure 2. A Dual Power Supply Circuit with External Clock Source
Figure 3. A Single Power Supply Circuit with Internal Clock Source
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 626A
DC ACCURATE, TUNABLE, LOW PASS FILTER
Figure 4. LTC1569-6/7 Block Diagram
LTC1569-6/7 Input and Output Considerations.
The filter' output (pin 8) can drive 10k and/or 40pF loads. On a DC626A, the filter' output is connected to VOUT1
s
s
(TP8) thru a 10Ω resistor so that the filter' output will not drive directly a capacitive load greater than 40pF
s
(capacitive loads greater than 40pF increase the output DC offset and AC distortion).
However, for convenience all DC626A measurements can be reliably made thru the buffered output at VOUT2.
Figures 2 and 3 show typical LTC1569-7 circuits using dual or single power supplies
(For an LTC1569-6, f
CUTOFF =
f
CLK
/64 otherwise the Figures 2 and 3 circuits are identical for an LTC1569-6).
If f
CUTOFF
0.5kHz for an LTC1569-6 or f
CUTOFF
1kHz for an LTC1569-7 (REXT 81k) then
an external clock is not needed for a fixed cutoff frequency (not clock tunable)
DC626A-A and DC626A-B Jumper Shunt Settings
JP1: DC or AC input signal coupling
JP2: DUAL or SINGLE supply operation
JP3: Connects an EXT CLK to pin 5 or sets pin 5 for divide by 1, 4 or 16 of the internal clock (set by REXT)
JP4: Pin 6 is set for an internal clock (INT) or an external clock (EXT)
JP5: Normally to OPEN, for measuring the filter' output DC voltage pin 1 is shorted to pin 3 (JP5 to SHORT).
s
JP6: Normally shorts the -IN input (pin 2) to FILTER AGND (pin 3) to provide for a single-ended to the +IN input (pin 1).
For a fixed cutoff frequency with a differential input, if the input impedance at IN- must be matched to the input imped-
ance at IN+ then connect optional resistor R3 from VIN- to FILTER AGND (remove shunt on JP6 shortingTP3 to TP4).
The value of R3 depends on the clock frequency and is equal to 62.5k• [64 kHz/f
CUTOFF
] for an LTC1569-6 or equal to
62.5k• [128 kHz/f
CUTOFF
] for an LTC1569-7 (see Figure 4 block diagram).
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