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DS51868A-page 10

© 2009 Microchip Technology Inc.

The MCP661 Line Driver Demo Board shows the MCP661 used in a very basic 
application for high speed op amps; a 50

Ω line (coax) driver. It gives:

• A 30 MHz solution

• High speed PCB layout techniques

• A means to test AC response, step response and distortion

The application circuit implemented on this PCB is discussed briefly in the 
MCP661/2/3/5 data sheet’s application circuit section.

Figure 1-2 shows a simplified circuit diagram of the MCP661 Line Driver Demo Board. 
Details of the power supply and connectors have been left out.

The 50

Ω source drives the matched 50Ω transmission line at the input; R

3

 provides this 

match. Thus, the input transmission line can be treated as a simple connection for 
circuit analysis (ignoring the time delay).

R

4

 provides matched input resistances for U

1

’s inputs. It also set a pole around 

100 MHz, since U

1

’s C

CM

 is about 9 pF.

U

1

 (MCP661) is set at a gain of 2 V/V so that the overall gain is 1 V/V (R

7

 and the Load 

attenuate the signal by 0.5 V/V). U

1

’s large output current makes it possible to drive the 

back-matched output transmission line (R

7

, the 50

Ω line and the 50Ω load at the far 

end) to more than ±2V (the load at the far end sees ±1V).

U

1

’s output headroom limits would be V

OL

= -2.3V and V

OH

= +2.3V, leaving some 

design room for the ±2V signal. The open-loop gain (A

OL

) typically does not decrease 

significantly with a 100

Ω load. The maximum power dissipated by the op amp is about 

48 mW, so the temperature rise (for the MCP661 in the SOIC-8 package) is under 8°C.

The output transmision line can be treated as a simple connection for circuit analysis 
(ignoring the time delay).

R

6

301

Ω

R

5

301

Ω

R

3

49.9

Ω

R

7

49.9

Ω

50

Ω

TX Line

V

OUT

(50

Ω Load)

V

IN

(50

Ω Source)

50

Ω

TX Line

U

1

R

4

124

Ω