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AD4001

Data Sheet

Rev. 0 | Page 20 of 33

High-Z Mode

The AD4001 incorporates high-Z mode, which reduces the

nonlinear charge kickback when the capacitor DAC switches

back to the input at the start of acquisition. Figure 40 shows the

input current of the AD4001 with high-Z mode enabled and

disabled. The low input current makes the ADC easier to drive

than the traditional SAR ADCs available in the market, even

with high-Z mode disabled. The input current reduces further

to sub microampere range when high-Z mode is enabled. The

high-Z mode is disabled by default but can be enabled by writing

to the register (see Table 14). Disable high-Z mode for input

frequencies above 100 kHz or multiplexing.

–12

–6

–9

–3

–15

–5

–1

–3

–2

–4

INPUT DIFFERENTIAL VOLTAGE (V)

25°C HIGH-Z ENABLED

25°C HIGH-Z DISABLED

Figure 40. Input Current vs. Input Differential Voltage,

VIO = 3.3 V, VREF = 5 V

System designers looking to achieve the optimum data sheet

performance from high resolution precision SAR ADCs are often

forced to use a dedicated high power, high speed amplifier to

drive the traditional switched capacitor SAR ADC inputs for

their precision applications, which is one of the common pain

points encountered in designing a precision data acquisition

signal chain. The benefits of high-Z mode are low input current

for slow (<10 kHz) or dc type signals and improved distortion

(THD) performance over a frequency range of up to 100 kHz.

High-Z mode allows a choice of lower power and bandwidth

precision amplifiers with a lower RC filter cutoff to drive the ADC,

removing the need for dedicated high speed ADC drivers, which

saves system power, size, and cost in precision, low bandwidth

applications. High-Z mode allows the amplifier and RC filter in

front of the ADC to be chosen based on the signal bandwidth of

interest and not based on the settling requirements of the

switched capacitor SAR ADC inputs.

Additionally, the AD4001 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 even larger

impedance. Figure 41 shows the THD performance for various

source impedances with high-Z mode disabled and enabled.

–125

–120

–115

–110

–105

–100

–95

–90

–85

–80

110

INPUT FREQUENCY (kHz)

150Ω HIGH-Z DISABLED

150Ω HIGH-Z ENABLED

510Ω HIGH-Z DISABLED

510Ω HIGH-Z ENABLED

1kΩ HIGH-Z DISABLED

1kΩ HIGH-Z ENABLED

Figure 41. THD vs. Input Frequency for Various Source Impedance, VREF = 5 V

Figure 42 and Figure 43 show the AD4001 SNR and THD

performance using the ADA4077-1 (IQUIESCENT = 400 μA/amplifier)

and ADA4610-1 (IQUIESCENT = 1.5 mA/amplifier) precision

amplifiers when driving the AD4001 at the full throughput of

2 MSPS for high-Z mode enabled and disabled with various RC

filter values. These amplifiers achieve 93 dB to 96 dB typical SNR

and better than −110 dB THD with high-Z mode enabled. THD

is approximately 10 dB better with high-Z mode enabled, even

for large R values. SNR holds up close to 96dB, even with a very

low RC bandwidth cutoff.

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

2 mW/MSPS extra power; however, this power 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

sheets of the selected precision amplifiers, shown in Figure 42

and Figure 43, that their own noise and distortion performance

dominates the SNR and THD specification at a certain input

frequency.

260kHz

1.3kΩ

470pF

498kHz

680Ω

470pF

1.3MHz

680Ω

180pF

2.27MHz

390Ω

180pF

4.42MHz

200Ω

180pF

RC FILTER BANDWIDTHS (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 42. SNR vs. RC Filter Bandwidth for Various Precision ADC Drivers,

VREF = 5 V, fIN = 1 kHz (Turbo Mode On, High-Z Enabled/Disabled), VIO = 3.3 V

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