AD8021AR Analog Devices Inc, AD8021AR Datasheet - Page 22

• Manufacturer Part Number: AD8021AR
• Description: IC OPAMP VF LN LP LDIST 8SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD8021AR

• Slew Rate: 460 V/µs
• Rohs Status: RoHS non-compliant
• Design Resources: Driving the AD7366/7 Bipolar SAR ADC in Low-Distortion DC-Coupled Appls (CN0042)
• Amplifier Type: Voltage Feedback
• Number Of Circuits: 1
• -3db Bandwidth: 560MHz
• Current - Input Bias: 7.5µA
• Voltage - Input Offset: 400µV
• Current - Supply: 7.8mA
• Current - Output / Channel: 70mA
• Voltage - Supply, Single/dual (±): 4.5 V ~ 24 V, ±2.25 V ~ 12 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• No. Of Amplifiers: 1
• Bandwidth: 560MHz
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Peak Reflow Compatible (260 C): No
• Input Bias Current: 7500nA
• Output Type: -
• Gain Bandwidth Product: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD8021
DRIVING 16-BIT ADCs
Low noise and adjustable compensation make the AD8021
especially suitable as a buffer/driver for high resolution ADCs.
As seen in Figure 19, the harmonic distortion is better than 90 dBc
at frequencies between 100 kHz and 1 MHz. This is an
advantage for complex waveforms that contain high frequency
information, because the phase and gain integrity of the sampled
waveform can be preserved throughout the conversion process.
The increase in loop gain results in improved output regulation
and lower noise when the converter input changes state during
a sample. This advantage is particularly apparent when using
16-bit high resolution ADCs with high sampling rates.
Figure 63 shows a typical ADC driver configuration. The
AD8021 is in an inverting gain of −7.5, f
output voltage is 10 V p-p. The results are listed in Table 7.
Table 7. Summary of ADC Driver Performance (f
V
Parameter
Second Harmonic Distortion
Third Harmonic Distortion
THD
SFDR
Figure 64 shows another ADC driver connection. The circuit
was tested with a noninverting gain of 10.1 and an output
voltage of approximately 20 V p-p for optimum resolution and
noise performance. No filtering was used. An FFT was
performed using Analog Devices evaluation software for the
AD7665
OUT
50Ω
= 10 V p-p)
Figure 64. Noninverting ADC Driver, Gain = 10, f
Figure 63. Inverting ADC Driver, Gain = −7.5, f
16-bit converter. The results are listed in Table 8.
50Ω
590Ω
200Ω
R
G
50Ω
82.5Ω
3
2
50Ω
R
+
–
AD8021
–12V
G
+12V
10pF
3
2
C
56pF
C
+
–
AD8021
+12V
–12V
1.5kΩ
OPTIONAL C
5
R
6
F
C
C
750Ω
R
5
F
IN
HI
IN
HI
Measurement
−101.3
−109.5
−100.0
+100.3
6
C
F
AD7665
570kSPS
is 65 kHz, and its
+5V
IN
HI
LO
IN
C
C
= 65 kHz
= 100 kHz
AD7665
570kSPS
ADC
C
+5V
= 65 kHz,
Unit
dBc
dBc
dBc
dBc
Rev. F | Page 22 of 28
Table 8. Summary of ADC Driver Performance
(f
Parameter
Second Harmonic Distortion
Third Harmonic Distortion
THD
SFDR
DIFFERENTIAL DRIVER
The AD8021 is uniquely suited as a low noise differential driver
for many ADCs, balanced lines, and other applications requiring
differential drive. If pairs of internally compensated op amps are
configured as inverter and follower, the noise gain of the inverter
is higher than that of the follower section, resulting in an
imbalance in the frequency response (see Figure 66).
A better solution takes advantage of the external compensation
feature of the AD8021. By reducing the C
inverter, its bandwidth can be increased to match that of the
follower, avoiding compromises in gain bandwidth and phase
delay. The inverting and noninverting bandwidths can be
closely matched using the compensation feature, thus
minimizing distortion.
Figure 65 illustrates an inverter-follower driver circuit operating
at a gain of 2, using individually compensated AD8021s. The
values of feedback and load resistors were selected to provide a
total load of less than 1 kΩ, and the equivalent resistances seen
at each op amp’s inputs were matched to minimize offset voltage
and drift. Figure 67 is a plot of the resulting ac responses of
driver halves.
C
= 100 kHz, V
V
49.9Ω
IN
OUT
Figure 65. Differential Amplifier
= 20 V p-p)
249Ω
499Ω
332Ω
232Ω
3
2
3
2
–V
–V
+
–
+
–
AD8021
AD8021
S
S
499Ω
664Ω
7pF
5pF
G = +2
G = –2
Measurement
−92.6
−86.4
−84.4
+5.4
5
5
6
6
COMP
1kΩ
1kΩ
value of the
V
OUT1
V
OUT2
Unit
dBc
dBc
dBc
dBc