Datasheet

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SBOS271D − MAY 2003 − REVISED JUNE 2007
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11
DRIVING CAPACITIVE LOADS
When using high-speed operational amplifiers, it is
extremely important to consider the effects of
capacitive loading on amplifier stability. Capacitive
loading will interact with the output impedance of the
operational amplifier, and depending on the capacitor
value, may significantly decrease the gain bandwidth,
as well as introduce peaking. To reduce the effects of
capacitive loading and allow for additional capacitive
load drive, place a series resistor between the output
and the load. This will reduce available bandwidth, but
permit stable operation with capacitive loading.
Figure 3 illustrates the recommended relationship
between the resistor and capacitor values.
Capacitive Load (pF)
1 10 100
100
75
50
25
0
Series Resistance (
Ω
)
Figure 3. Recommended R
S
and C
L
Combinations
Amplifiers configured in unity gain are most susceptible
to stability issues. The typical characteristic, Frequency
Response vs Capacitive Load, describes the relation-
ship between capacitive load and stability for the
OPA300/OPA301 series. In unity gain, the
OPA300/OPA301 series is capable of driving a few
picofarads of capacitive load without compromising
stability. Board level parasitic capacitance can often fall
into the range of a picofarad or more, and should be
minimized through good circuit-board layout practices
to avoid compromising the stability of the
OPA300/OPA301. For more information on detecting
parasitics during testing, see the Application Note
Measuring Board Parasitics in High-Speed Analog
Design (SBOA094), available at the TI web site
www.ti.com.
DRIVING A 16-BIT ADC
The OPA300/OPA301 series feature excellent
THD+noise, even at frequencies greater than 1MHz,
with a 16-bit settling time of 150ns. Figure 4 shows a
total single supply solution for high-speed data
acquisition. The OPA300/OPA301 directly drives the
ADS8401, a 1.25 mega sample per second (MSPS)
16-bit data converter. The OPA300/OPA301 is
configured in an inverting gain of 1, with a 5V single
supply. Results of the OPA300/OPA301 performance
are summarized in Table 1.
ADS8401
10
ΩV
IN
1.5nF
130pF
(mica)
OPA30x
1820
Ω
f
S
=1.25MSPS
f=10kHz
5V
130pF
(mica)
1820
Ω
Figure 4. The OPA30x Drives the 16-Bit ADS8401
PARAMETER RESULTS (f = 10kHz)
THD −99.3dB
SFDR 101.2dB
THD+N 84.2dB
SNR 84.3dB
Table 1. OPA30x Performance Results Driving a
1.25MSPS ADS8401