DN107 - C-Load TM Op Amps Conquer Instabilities

C-Load™ Op Amps Conquer Instabilities – Design Note 107
Kevin R. Hoskins
Introduction
Linear Technology Corporation has taken advantage of
advances in process technology and circuit innovations
to create a series of C-Load operational amplifiers that
are tolerant of capacitive loading, including the ultimate,
amplifiers that remain stable driving any capacitive
load. This series of amplifiers has a bandwidth that
ranges from 160kHz to 140MHz. These amplifiers are
appropriate for a wide range of applications from coaxial
cable drivers to analog-to-digital converter (ADC) input
buffer/amplifiers.
Driving ADCs
Most contemporary ADCs incorporate a sample-andhold (S/H). A typical S/H circuit is shown in Figure 1.
The hold capacitor’s (C1) size varies with the ADC’s
resolution but is generally in the range of 5pF to 20pF,
10pF to 30pF and 10pF to 50pF for 8-, 10- and 12-bit
ADCs, respectively.
an amplifier has not settled to a value that falls within
the error band of the ADC, conversion errors will result.
That is unless the amplifier is designed to gracefully
and accurately drive capacitive loads, such as Linear
Technology’s C-Load line of monolithic amplifiers.
Table 1 lists Linear Technology’s unconditionally stable
voltage feedback C-Load amplifiers. Table 2 lists other
voltage feedback C-Load amplifiers that are stable with
loads up to 10,000pF.
Table 1. Unity-Gain Stable C-Load Amplifiers Stable
with All Capacitive Loads
SINGLES
—
LT1200
LT1220
LT1224
LT1354
LT1357
LT1360
LT1363
DUALS
LT®1368
LT1201
—
LT1208
LT1355
LT1358
LT1361
LT1364
QUADS
LT1369
LT1202
—
LT1209
LT1356
LT1359
LT1362
LT1365
GBW
(MHz)
0.16
11
45
45
12
25
50
70
IS/AMP
(mA)
0.375
1
8
7
1
2
4
6
C1
>> –AV
AIN
TO SAR
GND
FROM INTERNAL DAC
DN107 • F01
Figure 1. Typical ADC Input Stage Showing
Input Capacitors
At the beginning of a conversion cycle, this circuit
samples the applied signal’s voltage magnitude and
stores it on its hold capacitor. Each time the switch
opens or closes, the amplifier driving the S/H’s input
faces a dynamically changing capacitive load. This
condition generates current spikes on the input signal.
This capacitive load and the spikes produced when
they are switched constitutes a very challenging load
that can potentially produce instabilities in an amplifier
driving the ADC’s input. These instabilities make it difficult for an amplifier to quickly settle. If the output of
07/95/107_conv
Table 2. Unity-Gain Stable C-Load Amplifiers Stable
with CL ≤ 10,000pF
SINGLES
LT1012
—
LT1097
—
DUALS
—
LT1112
—
LT1457
QUADS
—
LT1114
—
—
GBW
(MHz)
0.6
0.65
0.7
2
IS/AMP
(mA)
0.4
0.32
0.35
1.6
Remaining Stable in the Face of Difficult Loads
As can be seen in Figure 2, an amplifier whose design
is not optimized for handling a large capacitive load,
has some trouble driving the hold capacitor of the
LTC ®1410’s S/H. While the LT1006 has other very desirable characteristics such as very low VOS, very low
offset drift, and low power dissipation, it has difficulty
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and C-Load is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
accurately responding to dynamically changing capacitive loads and the current glitches and transients they
produce (as indicated by the instabilities that appear
in the lower trace of Figure 2a.
CONVST
1V/DIV
By contrast, Figure 2b shows the LT1360 C-Load op
amp driving the same LTC1410 input. The photo shows
that the LT1360 is an ideal solution for driving the
ADC’s input capacitor quickly and cleanly with excellent
stability. Its wide 50MHz gain-bandwidth and 800V/μs
slew rate very adequately complement the LTC1410’s
20MHz full power bandwidth. The LT1360 is specified
for ±5V operation.
Figure 3 shows the circuit used to test the performance
of op amps driving the LTC1410’s input and measure
the input waveforms.
INPUT
200mV/DIV
200ns/DIV
DN1007 F02a
Figure 2a. Input Signal Applied to an LTC1410
Driven by an LT1006
Conclusion
Linear Technology’s C-Load amplifiers meet the challenging and difficult capacitive loads of contemporary
ADC analog inputs by remaining stable and settling
quickly.
5V
0.1μF
CONVST
1V/DIV
INPUT
+
LT1006 OR
LT1360
+AIN
–
INPUT
50mV/DIV
12-BIT DATA
– AIN
0.1μF
LTC1410
– 5V
200ns/DIV
DN1007 F02b
AVDD
+
10μF
5V
0.1μF
0.1μF
DVDD
Figure 2b. Input Signal Applied to an LTC1410
Driven by an LT1360
AGND
OGND
CONTROL LINES
DGND
CONVST
DN107 • F03
Figure 3. Test Circuit Used to Measure LTC1410 Input
Signal Waveform
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn107f_conv LT/GP 0795 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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© LINEAR TECHNOLOGY CORPORATION 1995
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