Datasheet

V
OUT
V
IN
V
+
V
-
Q6
R
2
Q2
Q1
R
1
Q5
Q7
R
3
2:
R
4
2:
Q4
Q8
D2
D4
D6
D8D10 D12
D1
D3
D5
D7 D9
D11
Q3
LMH6321
www.ti.com SNOSAL8C –APRIL 2006–REVISED MARCH 2013
APPLICATION HINTS
BUFFERS
Buffers are often called voltage followers because they have largely unity voltage gain, thus the name has
generally come to mean a device that supplies current gain but no voltage gain. Buffers serve in applications
requiring isolation of source and load, i.e., high input impedance, low output impedance (high output current
drive). In addition, they offer gain flatness and wide bandwidth.
Most operational amplifiers, that meet the other given requirements in a particular application, can be configured
as buffers, though they are generally more complex and are, by and large, not optimized for unity gain operation.
The commercial buffer is a cost effective substitute for an op amp. Buffers serve several useful functions, either
in tandem with op amps or in standalone applications. As mentioned, their primary function is to isolate a high
impedance source from a low impedance load, since a high Z source can’t supply the needed current to the load.
For example, in the case where the signal source to an analog to digital converter is a sensor, it is recommended
that the sensor be isolated from the A/D converter. The use of a buffer ensures a low output impedance and
delivery of a stable output to the converter. In A/D converter applications buffers need to drive varying and
complex reactive loads.
Buffers come in two flavors: Open Loop and Closed Loop. While sacrificing the precision of some DC
characteristics, and generally displaying poorer gain linearity, open loop buffers offer lower cost and increased
bandwidth, along with less phase shift and propagation delay than do closed loop buffers. The LMH6321 is of the
open loop variety.
Figure 51 shows a simplified diagram of the LMH6321 topology, revealing the open loop complementary follower
design approach. Figure 52 shows the LMH6321 in a typical application, in this case, a 50Ω coaxial cable driver.
Figure 51. Simplified Schematic
SUPPLY BYPASSING
The method of supply bypassing is not critical for frequency stability of the buffer, and, for light loads, capacitor
values in the neighborhood of 1 nF to 10 nF are adequate. However, under fast slewing and large loads, large
transient currents are demanded of the power supplies, and when combined with any significant wiring
inductance, these currents can produce voltage transients. For example, the LMH6321 can slew typically at 1000
V/μs. Therefore, under a 50Ω load condition the load can demand current at a rate, di/dt, of 20 A/μs. This current
flowing in an inductance of 50 nH (approximately 1.5” of 22 gage wire) will produce a 1V transient. Thus, it is
recommended that solid tantalum capacitors of 5 μF to 10 μF, in parallel with a ceramic 0.1 μF capacitor be
added as close as possible to the device supply pins.
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