AD4020 datasheet(24/37 Pages) AD

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Data Sheet

AD4020

Rev. A | Page 23 of 36

Additionally, the AD4020 can be driven with a much higher

source impedance than traditional SARs, which means the

resistor in the RC filter can have a value 10 times larger than

previous SAR designs and, with high-Z mode enabled, can

tolerate an even larger impedance. Figure 42 shows the THD

performance for various source impedances with high-Z

mode disabled and enabled.

–85

–90

–100

–110

–95

–105

–115

–120

–125

INPUT FREQUENCY (KHz)

Ω HIGH-Z DISABLED

Ω HIGH-Z ENABLED

510

Ω HIGH-Z DISABLED

510

Ω HIGH-Z ENABLED

150

Ω HIGH-Z DISABLED

150

Ω HIGH-Z ENABLED

Figure 42. THD vs. Input Frequency for Various Source Impedances, VREF = 5 V

Figure 43 and Figure 44 show the AD4020 SNR and THD

performance using the ADA4077-1 (IQUIESCENT = 400 µA per

amplifier) and ADA4610-1 (IQUIESCENT = 1.5 mA per amplifier)

precision amplifiers when driving the AD4020 at the full

throughput of 1.8 MSPS for high-Z mode enabled and disabled

with various RC filter values. These amplifiers achieve +96 dB

to +99 dB typical SNR and better than −110 dB THD with high-Z

enabled. THD is approximately 10 dB better with high-Z mode

enabled, even for large R values. SNR maintains close to 99 dB,

even with a very low RC bandwidth cutoff.

100

260kHz

1.3k

Ω

470pF

498kHz

680

Ω

470pF

2.27MHz

390

Ω

180pF

1.3MHz

680

Ω

180pF

4.42MHz

200

Ω

180pF

RC FILTER BANDWIDTH (Hz)

RESISTOR (Ω), CAPACITOR (pF)

ADA4077-1 HIGH-Z DISABLED

ADA4077-1 HIGH-Z ENABLED

ADA4610-1 HIGH-Z DISABLED

ADA4610-1 HIGH-Z ENABLED

Figure 43. SNR vs. RC Filter Bandwidth for Various Precision ADC Drivers,

VREF = 5 V, fIN = 1 kHz (Turbo Mode On, High-Z Enabled/Disabled)

–80

–84

–92

–100

–88

–96

–104

–112

–108

–116

–120

260kHz

1.3k

Ω

470pF

498kHz

680

Ω

470pF

2.27MHz

390

Ω

180pF

1.3MHz

680

Ω

180pF

4.42MHz

200

Ω

180pF

RC FILTER BANDWIDTH (Hz)

RESISTOR (Ω), CAPACITOR (pF)

ADA4077-1 HIGH-Z DISABLED

ADA4077-1 HIGH-Z ENABLED

ADA4610-1 HIGH-Z DISABLED

ADA4610-1 HIGH-Z ENABLED

Figure 44. THD vs. RC Filter Bandwidth for Various Precision ADC Drivers,

VREF = 5 V, fIN = 1 kHz (Turbo Mode On, High-Z Enabled/Disabled)

When high-Z mode is enabled, the ADC consumes approximately

2.0 mW per MSPS extra power; however, this is still significantly

lower than using dedicated ADC drivers like the ADA4807-1.

For any system, the front end usually limits the overall ac/dc

performance of the signal chain. It is evident from the data

sheet of the selected precision amplifiers shown in Figure 43

and Figure 44 that their own noise and distortion performance

dominates the SNR and THD specification at a certain input

frequency.

Long Acquisition Phase

The AD4020 also features a very fast conversion time of 320 ns,

which results in a long acquisition phase. The acquisition is further

extended by a key feature of the AD4020; the ADC returns to the

acquisition phase typically 100 ns before the end of the

conversion. This feature provides an even longer time for the

ADC to acquire the new input voltage. A longer acquisition

phase reduces the settling requirement on the driving amplifier,

and a lower power/bandwidth amplifier can be chosen. The

longer acquisition phase means that a lower RC filter (represented

by R and C in Figure 31 to Figure 33 and Figure 36) cutoff

can be used, which means a noisier amplifier can also be

tolerated. A larger value of R can be used in the RC filter with

a corresponding smaller value of C, reducing amplifier stability

concerns without impacting distortion performance significantly.

A larger value of R also results in reduced dynamic power

dissipation in the amplifier.

See Table 10 for details on setting the RC filter bandwidth and

choosing a suitable amplifier.

VOLTAGE REFERENCE INPUT

A 10 µF (X7R, 0805 size) ceramic chip capacitor is appropriate

for the optimum performance of the reference input.

For higher performance and lower drift, use a reference such as

the ADR4550. Use a low power reference such as the ADR3450

at the expense of a slight decrease in the noise performance. It is

recommended to use a reference buffer, such as the ADA4807-1,

between the reference and the ADC reference input. It is important

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