User manual

MCP661 Line Driver Demo Board User’s Guide
DS51868A-page 10 © 2009 Microchip Technology Inc.
1.3 INTENDED USE
The MCP661 Line Driver Demo Board shows the MCP661 used in a very basic
application for high speed op amps; a 50Ω 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.
1.4 DESCRIPTION
1.4.1 Simplified Circuit
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.
FIGURE 1-2: Simplified Circuit Diagram.
The 50Ω 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 50Ω 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 100Ω 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
301Ω
R
5
301Ω
R
3
49.9Ω
R
7
49.9Ω
50Ω
TX Line
V
OUT
(50Ω Load)
V
IN
(50Ω Source)
50Ω
TX Line
U
1
MCP661
R
4
124Ω