Linear Technology 1251 User manual

DESCRIPTION

Demonstration Circuit DC1251A features the LTC6601-X

series family of pin configurable, fully differential, 2nd 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 2nd 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 2nd 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.

DEMO CIRCUIT 1251

QUICK START GUIDE

LTC6601-1, LTC6601-2

5MHz to 27MHz, Pin Configurable, Fully Differential, 2nd Order Lowpass Filter

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2

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).

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2 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

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2 ORDER LOWPASS FIL

TER

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.

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2 ORDER LOWPASS FIL

TER

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.

Figure 8. An LTC6601-1, Figure 5 circuit, configured as a 5MHz, lowpass filter with 0dB gain.

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2 ORDER LOWPASS FILTER

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2

ORDER LOWPASS FILTER

DEMO CIRCUIT 1251 QUICK START GUIDE

5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2

ORDER LOWPASS FILTER

DC1251A Parts List

Mouser Electronics

Authorized Distributor

Click to View Pricing, Inventory, Delivery & Lifecycle Information:

Analog Devices Inc.:

DC1251A-A DC1251A-B

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