DEMO CIRCUIT 1251
QUICK START GUIDE
LTC6601-1, LTC6601-2
5MHz to 27MHz, Pin Configurable, Fully Differential, 2
nd
Order Lowpass Filter
DESCRIPTION
Demonstration Circuit DC1251A features the LTC6601-X
series family of pin configurable, fully differential, 2
nd
or-
der lowpass filter and amplifier.
There are two DC1251A boards: a DC1251A-A with an
LTC6601-1 and a DC1251A-B with an LTC6601-2.
Typically an LTC6601-1 is the choice for most lowpass
filter applications in a 5MHz to 27MHz cutoff frequency
range and the LTC6601-2 is optimized for lower distor-
tion and lower power applications.
The LTC6601-X is designed to make the implementation
of high frequency fully-differential filtering functions very
easy. A very low noise amplifier is surrounded by 8 preci-
sion matched resistors and 12 precision matched capaci-
tors so that many 2
nd
order lowpass filter transfer func-
tions can be configured by hard wiring pins.
Figure 1 shows the LTC6601-X block diagram with all the
internal resistors and capacitors. On a DC1251A the
LTC6601-X resistor and capacitor pins can be connected
through 0603 resistor jumpers to configure the cutoff
frequency and the passband gain. The LTC6601-1 or
LTC6601-2 data sheet contains tables and circuits for
configuring the pins for a variety of cutoff frequencies
and passband gains (The cutoff or -3dB frequency is a
function of the fo frequency and Q value, refer to the Fig-
ure 3 filter equations in the LTC6601-1 data sheet).
Figure 2 shows an example of an LTC6601-1 configured
as a 2
nd
order lowpass filter with a 10MHz -3dB frequency
and a passband gain equal to 1 (0dB). The DC1251A-A/B
demonstration circuit default configuration is the circuit
of Figure 2.
The DC1251A has SMA connectors for the differential
input and output, a jumper with shunts to set the
LTC6601-X to high power (HP), low power (LP) or shut-
down (SHDN) operation and turrets to connect an exter-
nal VOCM. A DC1251A board operates with a single 2.7V
to 5.5V power supply.
Design files for these circuit boards are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Figure 1. An LTC6601-X Block Diagram.
Figure 2. The LTC6601-X Circuit as configured in a
DC1251A assembly.
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DEMO CIRCUIT 1251 QUICK START GUIDE
5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2
ND
ORDER LOWPASS FILTER
QUICK TEST SET-UP
Figure 3. A Quick Test Set-Up.
Quick Test Procedure
Test Equipment:
A Single Power Supply (3V – 5V).
A Sinewave Generator
An Oscilloscope (100MHz or greater).
1. Connect a DC1251 board as shown on Figure
1 (set the oscilloscope input impedance to
1MΩ).
2. Set the power supply to 3V.
3. Set the signal source output for a 1MHz,
1Vp-p sinewave.
4. Turn on the signal source and power supply.
5. Channel 1 and channel 2 on the oscilloscope
should each show a 1MHz, 0.5Vp-p sine-
wave.
Note 1: For accurate gain testing the DC1251 in-
puts must be ideally driven by a zero ohm source
(for example, the outputs of a high speed differ-
ential amplifier). However, the 50 ohm imped-
ance of the generator’s output and terminator is
in series with the LTC6601-X input resistors. A
50 ohm resistor in series with the input lowers
the expected passband gain and changes the fil-
ter’s Q value (refer to the Figure 3 GAIN and Q
equations in the LTC6601-1 data sheet).
2
DEMO CIRCUIT 1251 QUICK START GUIDE
5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2
ND
ORDER LOWPASS FILTER
CONFIGURING A DC1251 BOARD
The basic filter topology shown in Figure 4, can im-
plement most LTC6601-X lowpass filter circuits. A
DC1251A board has all the connections required for
configuring the LTC6601-X basic filter topology. An
example of a basic filter circuit and its DC1251A con-
figuration is shown in Figure 6 and Figure 7 respec-
tively.
The modified filter topology shown in Figure 4 pro-
vides for additional lowpass filter options. The use of
the REXT resistor provides for setting precisely the
filter’s Q value without changing the fo frequency. In
addition, the use of REXT provides for setting the
filter’s gain to greater than one without changing the
Q value (refer to the Figure 13 Q equation in the
LTC6601-1 data sheet).
The modified topology’s C3 (the capacitor between
the summing junction) implements lower Q values
than the basic filter topology using only C1 and C2
(and lower Q values provide the option to implement
lower -3dB frequencies).
A DC 1251A board has resistors pads for connecting
the REXT of the modified topology. However the
DC1251A layout does not have surface mount pads
for connecting the 49.9 ohm resistor of the modified
topology. An example of a modified filter circuit and
its DC1251A configuration is shown in Figure 8 and
Figure 9 respectively.
Note: The range of values for the resistor that con-
nects the capacitors of the modified filter topology is
25–50 ohms.
Figure 4. The LTC6601-X basic filter topology.
Figure 5. The LTC6601-X modified filter topology
3
DEMO CIRCUIT 1251 QUICK START GUIDE
5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2
ND
ORDER LOWPASS FILTER
Figure 6. An LTC6601-1, Figure 4 circuit, configured as a 7MHz, lowpass filter with 12dB gain.
Figure 7. DC1251A configured as the Figure 6 circuit.
RIN1, RIN2, RG3, RG4, RF3-RF6, RF9-RF12 are zero ohm, 0603 resistors.
For DC coupling, RZ1 and RZ2 are zero ohm resistors.
For AC coupling, RZ1 and RZ1 are 0.1uF capacitors.
4
DEMO CIRCUIT 1251 QUICK START GUIDE
5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2
ND
ORDER LOWPASS FILTER
Figure 8. An LTC6601-1, Figure 5 circuit, configured as a 5MHz, lowpass filter with 0dB gain.
Figure 9. DC1251A configured as the Figure 8 circuit.
RIN1, RIN2, RG1, RG6, RF5-RF6, RF11 and RF12 are zero ohm, 0603 resistors.
RQ3 is 150 ohms, 0603 resistor.
For the 49.9 ohm resistor connection, short the RF3, RF4 and RF9, RF10 pads
next to U1 and connect a 49.9 leaded resistor between the shorted pads.
For DC coupling, RZ1 and RZ2 are zero ohm resistors.
For AC coupling, RZ1 and RZ1 are 0.1uF capacitors.
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