APPLICATION INFORMATION

WIDEBAND BUFFER OPERATION

V

CC

To V

CC/2

BUF602

2k

Ω

50

Ω

0.1

µF

50

ΩLoad

V

OUT

V

CC/2

0.1

µF

200

Ω

50

Ω

LOW-IMPEDANCE TRANSMISSION LINES

+

50

Ω

BUF602

50

Ω

V

IN

50

ΩSource

V

OUT

50

ΩLoad

0.1

µF

4.7

µF

−5V

+

0.1

µF

4.7

µF

+5V

Z =

O

LT

CT

(1)

BUF602

www.ti.com ...................................................................................................................................................... SBOS339B – OCTOBER 2005 – REVISED MAY 2008

single-supply operation of the BUF602 is to maintain

output signal swings within the usable voltage ranges.

The circuit of Figure 32 establishes an input midpoint

The BUF602 gives the exceptional AC performance

bias using the internal midpoint reference. The input

of a wideband buffer. Requiring only 5.8mA quiescent

signal is then AC-coupled into this midpoint voltage

current, the BUF602 will swing to within 1V of either

bias. Again, on a single +5V supply, the output

supply rail and deliver in excess of 60mA at room

voltage can swing to within 1V of either supply pin

temperature. This low output headroom requirement,

while delivering more than 60mA output current. A

along with supply voltage independent biasing, gives

demanding 100

Ω load to a midpoint bias is used in

remarkable

single

(+5V)

supply

operation.

The

this characterization circuit.

BUF602 will deliver greater than 500MHz bandwidth

driving a 2VPP output into 100Ω on a single +5V

supply.

Figure 31 shows the DC-coupled, dual power-supply

circuit configuration used as the basis of the ±5V

Electrical

and

Typical

Characteristics.

For

test

purposes, the input impedance is set to 50

Ω with a

resistor to ground and the output impedance is set to

50

Ω with a series output resistor. Voltage swings

reported in the specifications are taken directly at the

input and output pins while load powers (dBm) are

defined at a matched 50

Ω load. In addition to the

usual power-supply decoupling capacitors to ground,

a 0.01

µF capacitor can be included between the two

power-supply pins. This optional added capacitor will

typically

improve

the

2nd-harmonic

distortion

performance by 3dB to 6dB.

Figure 32. AC-Coupled, Single-Supply,

Specification and Test Circuit

The most important equations and technical basics of

transmission lines support the results found for the

various drive circuits presented here. An ideal

transmission medium with zero ohmic impedance

would have inductance and capacitance distributed

over the transmission cable. Both inductance and

capacitance detract from the transmission quality of a

line. Each input is connected with high-impedance to

the

line

as

in

a

daisy-chain

or

loop-through

Figure 31. DC-Coupled, Bipolar Supply,

configuration, and each adds capacitance of at least

Specification and Test Circuit

a few picofarads. The typical transmission line

impedance (ZO) defines the line type. In Equation 1,

Figure 32 shows the AC-coupled, single-supply circuit

the impedance is calculated by the square root of line

configuration used as the basis of the +5V Electrical

inductance (LT) divided by line capacitance (CT):

and Typical Characteristics. Though not a rail-to-rail

design, the BUF602 requires minimal input and

output voltage headroom compared to other very

wideband buffers. It will deliver a 3VPP output swing

on a single +5V supply with greater than 400MHz

bandwidth.

The

key

requirement

of

broadband

Copyright © 2005–2008, Texas Instruments Incorporated

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