AD4003 Datasheet, PDF (21/34 Pages) Analog Devices – 18-Bit, 2 MSPS Precision SAR Differential ADC

AD4003
Data Sheet
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.
15
25°C HIGH-Z ENABLED
12
25°C HIGH-Z DISABLED
9
6
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
–95
–100
–105
–110
–115
1kΩ HIGH-Z DISABLED
1kΩ HIGH-Z ENABLED
510Ω HIGH-Z DISABLED
510Ω HIGH-Z ENABLED
3
–120
0
150Ω HIGH-Z DISABLED
150Ω HIGH-Z ENABLED
–125
–3
1
10
20
INPUT FREQUENCY (KHz)
–6
Figure 41. THD vs. Input Frequency for Various Source Impedance, VREF = 5 V
–9
–12
–15
–5 –4 –3 –2 –1 0 1 2 3 4 5
INPUT DIFFERENTIAL VOLTAGE (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
Figure 40. Input Current vs. Input Differential Voltage,
VIO = 3.3 V, VREF = 5 V
of the signal chain. It is evident from the data sheet of the
selected precision amplifiers shown in Figure 42 and Figure 43
System designers looking to achieve the optimum data sheet
that their own noise and distortion performance dominates the
performance from high resolution precision SAR ADCs are often
SNR and THD specification at a certain input frequency.
forced to use a dedicated high power, high speed amplifier to
100
drive the traditional switched capacitor SAR ADC inputs for
97
their precision applications, which is one of the common pain
94
points encountered in designing a precision data acquisition
signal chain. The benefits of high-Z mode are low input current
91
for slow (<10 kHz) or dc type signals and improved distortion
88
(THD) performance over a frequency up to 100 kHz. High-Z
85
mode allows a choice of lower power and bandwidth precision
82
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.
79
76
73
70
260kHz
1.3kΩ
470pF
ADA4077-1 HIGH-Z DISABLED
ADA4077-1 HIGH-Z ENABLED
ADA4610-1 HIGH-Z DISABLED
ADA4610-1 HIGH-Z ENABLED
498kHz
680Ω
470pF
1.3MHz
680Ω
180pF
2.27MHz
390Ω
180pF
4.42MHz
200Ω
180pF
RC FILTER BANDWIDTH (Hz)
RESISTOR (Ω), CAPACITOR (pF)
Additionally, the AD4003 can be driven with a much higher source
impedance than traditional SARs, which means the resistor in the
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)
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
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