BUF04 Datasheet, PDF (10/16 Pages) Analog Devices – Closed-Loop High Speed Buffer

BUF04
A two-terminal equivalent circuit of the BUF04 is shown in
Figure 30 where the transistor-level equivalent circuit is reduced
to its essential elements. The input stage develops a signal
current, I_IN, that is replicated by an internal current conveyor so
as to flow through R_t, the transimpedance of the BUF04. The
voltage developed across R_t is buffered by a unity-gain output
voltage follower. With an open-loop R_t of 400 kΩ and an R_IN of
30 Ω, the voltage gain of the BUF04, given by the ratio R_t/R_IN is
approximately 13,000—accurate to approximately 13.5 bits.
The BUF04’s open-loop ac transimpedance response is
determined by the open-loop pole formed by R_t and C_t. Since
C_t is typically 8 pF, the open-loop pole occurs at approximately
50 kHz.
V_IN
X1
I_IN
R_{t Ct}
XI
I_IN
R_IN
V_OUT
R_FB
R_IN = 30 Ω
R_t = 400 k Ω
C_t = 8pF
RFB = 100Ω
Figure 30. Current-Feedback Functional Equivalent
Circuit of the BUF04
Grounding and Bypassing Considerations
To take full advantage of the BUF04’s very wide bandwidth,
high slew rates, and dynamic range capabilities requires due
diligence with regard to supply bypassing. In high speed circuits,
the supply bypassing network must provide a very low impedance
return path for currents flowing to and from the load network.
As with any high speed application, multiple bypassing is always
recommended. A 10 µF tantalum electrolytic in parallel with a
0.1 µF ceramic capacitor is sufficient for most applications. For
those high speed applications where output load currents
approach 50 mA, small valued resistors (1.1 Ω to 4.7 Ω) in
series with the tantalum capacitors may improve circuit
transient response by damping out the capacitor’s self-
inductance. Figure 31 illustrates bypassing recommendations.
V+ _{10µF R1}
0.1µF
KELVIN RETURN
FOR LOAD CURRENT
7
V_IN
3
6
BUF04
V_OUT
R_S
4
R_L
0.1µF
10µF _R2
KELVIN RETURN
V–
FOR LOAD CURRENT
NOTE
USE SHORT LEAD LENGTHS (<5mm)
Figure 31. Recommended Power-Supply Bypassing
To minimize the effects of high-frequency coupling, circuits
must be built with short interconnect leads, and large ground
planes should he used whenever possible to provide a low
resistance, low-inductance circuit path. Sockets should be
avoided because the increased interlead capacitance can degrade
bandwidth and stability. If sockets are necessary, individual pin
sockets (oftentimes called “cage jacks,” AMP Part No.
5-330808-3 or 5-330808-6) should be used. They contribute far
less stray reactance than molded socket assemblies.
Offset Voltage Nulling
Although the offset voltage of the BUF04 is very low (1 mV,
maximum) for such a high speed buffer, the circuit shown in
Figure 32 can be used if additional offset voltage nulling is
required. A potentiometer ranging from 1 k to 10 k can be used
for V_OS nulling; with a 10 kΩ potentiometer, the trim range is
± 30 mV.
V+
10µF
TRIM RANGE
±30mV
1 _10k
8
V_IN
3 _BUF04
4
0.1µF
7
6
0.1µF
V_OUT
10µF
V–
Figure 32. Optional Offset Voltage Nulling Scheme
APPLICATIONS
Output Short-Circuit Protection
To optimize the transient response and output voltage swing of
the BUF04, internal output short-circuit current limiting was
omitted. Although the BUF04 can provide continuous output
currents of 50 mA without protection, direct connection of the
BUF04’s output to ground or to the supplies will destroy the
device. An active current limit technique, illustrated in Figure
33, provides the necessary short-circuit protection while
retaining full dc output voltage swing to the load.
+15V
10µF
RSC1
≥10Ω
2N2905
2N2905
0.1µF
7
V_IN ³ _BUF04
6
SET ISC +(ISC–) <60mA,
CONTINUOUS
RSC1 (RSC2) =
0.6V
ISC + (ISC–)
V_OUT
0.1µF
4
6.2k Ω _0.01µF
2N2219
RSC2
≥10Ω
2N2219
10µF
–15V
Figure 33. Short-Circuit Current Limiting Using
Current Sources
–10–
REV. 0