BUF602 Datasheet, PDF (14/22 Pages) Texas Instruments – High-Speed, Closed-Loop Buffer

BUF602
SBOS339 – OCTOBER 2005
APPLICATION INFORMATION
WIDEBAND BUFFER OPERATION
The BUF602 gives the exceptional AC performance
of a wideband buffer. Requiring only 5.8mA quiescent
current, the BUF602 will swing to within 1V of either
supply rail and deliver in excess of 60mA at room
temperature. This low output headroom requirement,
along with supply voltage independent biasing, gives
remarkable single (+5V) supply operation. The
BUF602 will deliver greater than 500MHz bandwidth
driving a 2V_PP output into 100Ω on a single +5V
supply.
Figure 29 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 50Ω with a
resistor to ground and the output impedance is set to
50Ω 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 50Ω load. In addition to the
usual power-supply decoupling capacitors to ground,
a 0.01µF 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.
+5V
0.1µF ⁺ 4.7µF
50Ω Source
V_IN
BUF602
50Ω
50Ω
V_OUT
50Ω Load
0.1µF ₊ 4.7µF
−5V
Figure 29. DC-Coupled, Bipolar Supply,
Specification and Test Circuit
Figure 30 shows the AC-coupled, single-supply circuit
configuration used as the basis of the +5V Electrical
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 3V_PP output swing
on a single +5V supply with greater than 400MHz
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bandwidth. The key requirement of broadband
single-supply operation of the BUF602 is to maintain
output signal swings within the usable voltage ranges.
The circuit of Figure 30 establishes an input midpoint
bias using the internal mid-point reference. The input
signal is then AC-coupled into this mid-point voltage
bias. Again, on a single +5V supply, the output
voltage can swing to within 1V of either supply pin
while delivering more than 60mA output current. A
demanding 100Ω load to a mid-point bias is used in
this characterization circuit.
V_CC
0.1µF
50Ω
2kΩ
200Ω
0.1µF
BUF602
50Ω
50Ω Load
V_OUT
To V_CC/2
V_CC/2
Figure 30. AC-Coupled, Single-Supply,
Specification and Test Circuit
LOW-IMPEDANCE TRANSMISSION LINES
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
configuration, and each adds capacitance of at least
a few picofarad. The typical transmission line
impedance (Z_O) defines the line type. In Equation 1,
the impedance is calculated by the square root of line
inductance (L_T) divided by line capacitance ©_T):
Ǹ Z₀ +
L_T
C_T
(1)
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