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AD4003

Data Sheet

Rev. 0 | Page 20 of 33

High-Z Mode

The AD4003 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 AD4003 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 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 AD4003 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 40 shows the THD performance for various

source impedances with high-Z mode disabled and enabled.

Figure 42 and Figure 43 show the AD4003 SNR and THD perfor-

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

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

when driving the AD4003 at the full throughput of 2 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 holds up close to 99 dB, even with a very low RC

bandwidth cutoff.

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

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

2 mW/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 42 and Figure 43

that their own noise and distortion performance dominates the

SNR and THD specification at a certain input frequency.

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

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

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