BUF04 Datasheet, PDF (11/16 Pages) Analog Devices – Closed-Loop High Speed Buffer

BUF04
Output Current Transient Recovery
Settling characteristics of high speed buffers also include the
buffer’s ability to recover, i.e., settle, from a transient output
current load condition. When driving the input of an A/D
converter, especially the successive-approximation converter
types, the buffer must maintain a constant output voltage under
dynamically changing load current conditions. In these types of
converters, the comparison point is usually diode-clamped, but
it may deviate several hundred millivolts resulting in high
frequency modulation of the A/D input current. Open-loop and
closed-loop buffers (also, op amps configured as followers) that
exhibit high closed-loop output impedances and/or low unity
gain crossover frequencies recover very slowly from output load
current transients. This slow recovery leads to linearity errors or
missing codes because of errors in the instantaneous input volt-
age. Therefore, the buffer (or op amp) chosen for this type of
application should exhibit low output impedance and high unity
gain bandwidth so that its output has had a chance to settle to
its nominal value before the converter makes its comparison.
The circuit in Figure 34 illustrates a settling measurement
circuit for evaluating the recovery time of high speed buffers
from an output load current transient. The input to the buffer is
grounded for ease of measuring the recovery time, and two
resistors are used to sum steady-state and transient load currents
at the output. As a worst-case condition, R1, was chosen such
that the BUF04 would source (or sink) a steady-state current of
25 mA. R2 was then chosen to add a 10 mA transient current
upon the steady-state value. To set accurately the nodal voltages
internal to the BUF04, the supply voltages were offset by the
voltage applied to R1. Because of its high transimpedance, wide
bandwidth, and low output impedance, the BUF04 exhibits an
extremely fast recovery time of 60 ns to 0.01%, as shown in
Figure 34. Results were identical regardless whether the BUF04
was sourcing or sinking current.
V+
10µF
0.1µF
TP1
TP2
7
R2
3 _BUF04 6
250Ω
V_IN
0.1µF
4
R1
200Ω
SOURCE: 0 –2.5 V
SINK: 0 +2.5V
10µF ^VLOAD
SOURCE: –5V
SINK: +5V
V–
Figure 34. Transient Output Load Current Test Circuit
I_{SOURCE 100}
(4mA/DIV) ₉₀
∆^t 59.00ns
25mA
35mA
V_OUT 10
(5mV/DIV) ^0%
100mV
5mV
20ns
Figure 35. BUF04’s Output Load Current Recovery Time
Terminated Line Drivers
The BUF04’s high output current, large slew rate, and wide
bandwidth all combine to make it an ideal device for high speed
line driver applications. As shown in Figure 36, the BUF04 can
be configured for driving doubly terminated 50 Ω and 75 Ω
cables. To optimize the circuit’s pulse response, a capacitor, C_T
(C_X + C_TRIM), is connected across the series back termination.
The BUF04 can drive a 50 Ω line to ± 2.5 V and a 75 Ω line to
± 3.75 V when operating on ± 15 V supplies.
C_T
C_X
6'
V_IN
3 _BUF04 6
R_X
COAX
R_S
R_L
Z_O COAX R_S, R_L R_X
50Ω RG-58 50Ω 50
75Ω RG-59 75Ω 75
C_X
91pF
62pF
C_T
3–15pF
3–15pF
Figure 36. Line Driver Configuration
Low-Pass Active Filter
In many signal-conditioning applications, filters are required to
band-limit noise or altogether eliminate other unwanted signals
prior to conversion. Often, high frequency filters are needed for
these applications; however, there are few op amps that exhibit
the high open-loop gain and wide unity-gain crossover
frequency required for these applications. As illustrated in
Figure 37, the BUF04 and a handful of passive components can
be configured as a high frequency, low-pass active filter. Since
the filter configuration is a unity-gain Sallen-Key topology, the
BUF04 is particularly well suited for this application. In this
circuit, an additional resistor, R3, was added to prevent
interaction between C2 and the BUF04’s input capacitance.
C1*
44pF (22pF x 2)
R1
V_IN
499Ω
R2
499Ω
C2*
22pF
R3
47Ω ₃
6
BUF04
V_OUT
* SILVERED MICA OR
DIPPED CERAMIC
1
W_O = R1 · R2 · C1 · C2 ; Q =
C1
4 · C2
Figure 37. A 10 MHz Low-Pass Active Filter
REV. 0
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