BUF04

REV. 0

–11–

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.

BUF04

7

6

0.1µF

0.1µF

10µF

4

3

TP2

TP1

R2

250

Ω

10µF

R1

200

Ω

V+

SOURCE: –5V

SINK: +5V

SOURCE: 0  –2.5 V

SINK: 0  +2.5V

V–

Figure 34. Transient Output Load Current Test Circuit

10

90

100

0%

5mV

59.00ns

20ns

100mV

(4mA/DIV)

(5mV/DIV)

35mA

25mA

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

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.

6

3

6'

COAX

BUF04

50

Ω

75

Ω

COAX

RG-58

RG-59

50

Ω

75

Ω

50

75

91pF

62pF

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.

BUF04

6

3

R1

499

Ω

R2

499

Ω

R3

47

Ω

C1*

44pF (22pF x 2)

C2*

22pF

* SILVERED MICA OR

DIPPED CERAMIC

R1 · R2 · C1 · C2

1

;

Q =

4 · C2

C1

Figure 37. A 10 MHz Low-Pass Active Filter