CN0385 Datasheet, PDF (11/13 Pages) Analog Devices – Devices Connected

Circuit Note
CN-0385
The mean code error increases as the voltage step size and
channel switching rate increase. This increase is caused by the
combined slew and settling time limitations of the components
in the signal chain. Increasing the step size forces the system to
settle larger changes in voltage and increasing the channel
switching rate decreases the amount of time the system is given
to settle these changes. At sufficiently high step sizes and
switching rates, the mean code error becomes unpredictably
large, as in the gain of 0.4 configuration (see Figure 17 and
Figure 21). This code error is caused by the slew rate limitations
of the input buffer amplifiers in the AD8251 in-amp.
The performance of the system when using the ac source was
evaluated by comparing its THD with respect to the channel
switching rate. The AP SYS-2712 provided a full-scale sine wave
input on one channel and an inverted version of the sine wave
on another channel. THD was measured for various sample
rates, ranging from 25 kSPS to 1.5 MSPS in 25 kSPS increments.
Figure 25 shows the THD measured for each of the channels in
each of the gain configurations.
–40
GAIN = 0.4dB
GAIN = 0.8dB
–50
GAIN = 1.6dB
GAIN = 3.2dB
–60
–70
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0385-FMCZ circuit board and
the EVAL-SDP-CH1Z SDP-H1 system demonstration platform
controller board. The two boards have 160-pin mating connectors,
allowing quick setup and evaluation of the performance of the
circuit. The circuit board contains the circuit to be evaluated, as
described in this circuit note, and the SDP-H1 controller board
is used with the CN-0385 Evaluation Software to capture the data
from the circuit board.
Equipment Needed
The following equipment is needed:
 PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows 7 (32-bit)
 EVAL-CN0385-FMCZ circuit evaluation board
 EVAL-SDP-CH1Z SDP-H1 controller board
 CN-0385 Evaluation Software (download from
ftp://ftp.analog.com/pub/cftl/CN0385/)
 5 V to 12 V dc power supply or wall wart
(9 V wall wart included with EVAL-CN0385-FMCZ board)
 USB to micro-USB cable
 Low distortion, low output impedance signal generator to
provide ±10 V output
 Low noise, high precision dc supply to provide ±10 V output
Getting Started
–80
–90
–100
–110
0
200
400
600
800
CHANNEL SWITCHING RATE (kHz)
Figure 25. THD vs. ADG5207 Channel Switching Rate for Full-Scale
1 kHz Input
Download the evaluation software from ftp://ftp.analog.com/
pub/cftl/CN0385/, and then install the software on the PC.
Functional Block Diagram
See Figure 1 for the circuit block diagram and the EVAL-CN0385-
FMCZ-SCH.pdf file for the complete circuit schematic. This file
is contained in the CN-0385 Design Support Package (available
at www.analog.com/CN0385-DesignSupport). A functional block
diagram of the test setup is shown in Figure 26.
The THD performance of the system begins to degrade at
roughly 700 kSPS (depending on the gain configuration). The
whole signal chain SNR and THD performance is mainly
limited by the PGA AD8251. A smaller RC filter at the front of
AD4003 also gives better THD at higher channel switching
sampling rate.
COMMON VARIATIONS
5V TO 12V
POWER
PC
AC/DC
SOURCE(S)
J1/P3
P1
EVAL-CN0385-CH1Z
USB
P2/
P8
EVAL-SDP-CH1Z
The AD4003 ADC is pin-for-pin compatible with various
other 14-bit, 16-bit, and 18-bit, 10-lead precision SAR ADCs
that can be used in the CN-0385 system. The ADG1207, with
wider bandwidth, can be an alternative to the ADG5207. The
ADG5248F, with fault protection and detection function, can
be used for single-ended inputs. The AD8475 provides a
differential output signal for other differential ADCs, such as
the AD7690. The ADA4805-1 op amp is an alternative for the
AD8475 when driving pseudo differential or single-ended
ADCs, such as the AD4000. Other Analog Devices, Inc., LDOs,
such as the ADP7102 and ADP7142, can replace the ADP7118.
Figure 26. Test Setup Functional Block Diagram
Hardware Setup
Figure 27 shows the EVAL-CN0385-FMCZ evaluation hardware.
Information and details regarding the SDP-H1 board can be
found in the SDP-H1 User Guide.
Connect the 160-pin connector on the circuit board to the J4
connector on the SDP-H1 controller board.
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