EVAL-CN0410-ARDZ - ANALOG DEVICES

Circuit Note

CN-0410

Circuits from the Lab® reference designs are engineered and

tested for quick and easy system integration to help solve today’s

analog, mixed-signal, and RF design challenges. For more

information and/or support, visit www.analog.com/CN0410.

Devices Connected/Referenced

AD5686A Serial-Input, Voltage Output, Unbuffered

16-Bit DAC

ADA4500-2 Rail-to-Rail Input/Output, Zero Input

Crossover Distortion Amplifier

ADR4520 Ultralow Noise, High Accuracy, 2.048 V

Voltage Reference

LTC6820 Isolated SPI Communication Interface

ADP7112 20 V, 200 mA Linear Regulator

Programmable, 3-Channel LED Current Source Driver with isoSPI Repeater

Rev. 0

Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog

Devices engineers. Standard engineering practices have been employed in the design and

construction of each circuit, and their function and performance have been tested and verified in a lab

environment at room temperature. However, you are solely responsible for testing the circuit and

determining its suitability and applicability for your use and application. Accordingly, in no event shall

Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due

to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0410 Circuit Evaluation Board (EVAL-CN0410-ARDZ)

ADICUP3029 (EVAL-ADICUP3029)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 is a 3-channel, programmable,

LED current driver with digital communications repeater

cababilites.

Each of the three output channels is controlled by a single

digital-to-analog converter (DAC), which simplifies the digital

communications and reduces cost. Each channel can be

individually programmed and is capable of sourcing 1 A of

current through its load.The circuit also has an isoSPI™ repeater

that allows multiple boards to be controlled with a single

master. The repeater translates the serial port interface (SPI)

signals into pulses that are transmitted through a twisted pair of

wire and converted back to an SPI signal at the other end.

This solution is ideal for precision lighting control applications

where a compact, scalable, single-supply, and highly linear,

3-channel, 16-bit buffered current source is required.

CN-0410 Circuit Note

Rev. 0 | Page 2 of 7

Figure 1. Linear 16-Bit Buffered Triple Output Voltage Output DAC (Simplified Schematic, All Connections and Decoupling Not Shown)

CIRCUIT DESCRIPTION

Digital-to-Analog Converter Output

Figure 1 shows the single-supply signal chain that consists of a

voltage reference, a DAC, a DAC buffer, voltage to current

conversion stage, and an isolated SPI communication interface.

The DAC is the AD5686, a low power, quad, 16-bit, buffered

voltage output DAC. The output voltage of the DAC is dependent

on the reference voltage, as shown in the following equation:

REF

OUT DV





where

D is the decimal data word loaded in the DAC register.

N is the number of bits.

For a reference of 2.048 V, and N = 16, the equation simplifies

to the following:

VOUT =(2.048 × D)/65536

This equation gives a VOUT of 1.024 V at mid scale, and 2.048 V

at full scale.

The LSB size is 2.048 V/65,536 = 31.25 μV.

One LSB at 16 bits is also 0.0015% of full scale or 15 ppm of full

scale.

The DAC reference pin is driven by a 2.0248 V reference device,

the ADR4520B. The ADR4520B voltage reference provides a

high precision, low noise (1 μV p-p, 0.1 Hz to 10 Hz) and stable

reference to the DAC. The ADR4520B uses an innovative core

topology to achieve high accuracy, temperature stability, and

noise performance. The low output voltage temperature

coefficient (2 ppm/°C maximum) and low long-term output

voltage drift of the device also improve system accuracy over

time and temperature variations. The initial room temperature

accuracy of the ADR4520B is ±0.02% maximum, which is

approximately 13 LSBs at 16 bits.

Voltage to Current Conversion

The ADA4500-2 is selected as the operational amplifier. This

device is a high precision amplifier with a maximum offset

voltage of 120 μV, offset drift of less than 5.5 μV/°C, 0.1 Hz to

10 Hz noise of 2 μV p-p, and maximum input bias current of

2 pA. Its key feature of rail-to-rail input and output swing with

zero crossover distortion makes the ADA4500-2 a suitable

candidate for this function.

A typical rail-to-rail input amplifier uses two differential pairs

to achieve rail-to-rail input swing (see the MT-035 Tutorial).

One differential pair is active at the higher range of the input

common-mode voltage, and the other pair is active at the lower

end. This classic dual differential pair topology introduces

crossover distortion during the handoff of one differential pair

to the other. The change in offset voltage causes nonlinearity

when the amplifier is used as a DAC buffer. The ADA4500-2

uses an integrated charge pump in its input structure to achieve

ADP7112 ADR4520 ADA4500-2

AD5686

ADA4500-2

VIN

9V TO 20V +3.3V +2.048

+3.3V

isoSPI

SCLK

GND

MOSI

SYNC

10µF

0.1µF

GND

VOUT V

REF

OUT

GND

OUT

SERIAL

INTERFACE

LTC6820

SCLK

1:1

MOSI

SYNC

2Ω

1nF

1kΩ

2.2µF

ADA4500-2

2Ω

1nF

1kΩ

2.2µF

ADA4500-2

2Ω

1nF

1kΩ

2.2µF

16405-001

Circuit Note CN-0410

Rev. 0 | Page 3 of 7

rail-to-rail input swing without the need for a second

differential pair. Therefore, it does not exhibit crossover

distortion. Using a zero crossover distortion amplifier in this

single-supply system provides wide dynamic output range while

maintaining linearity over the input common mode/input

digital code range. Details of the operation of the ADA4500-2

can be found in the ADA4500-2 data sheet.

The ADA4500-2 is a suitable candidate with high input impedance,

2 pA maximum input bias current at room temperature, and

190 pA maximum input bias current over temperature. This low

bias current results in 1.2 μV of worst-case error due to input

bias current, which is much less than 1 LSB.

The output the DAC is buffered and used to turn on the

MOSFET, with feedback taken from the current sense resistor.

A current source must be compensated properly to prevent

oscillations when driving an inductive load such as the wiring

to the LED board. The Rx resistor, Ry, resistor, Cx capacitor, and

Cy output capacitor provide frequency compensation. This

circuit tolerates a load inductance of up to 5 μH. (For example,

an LED board wired to the CN-0410, located 5 m away, with 16-

gauge wire, spaced 10 mm apart, is approximately 5.5 μH. Most

practical installations have conductors much closer together,

and therefore lower inductance.) An LTspice® simulation is

provided to aid in compensating for other loads.

Figure 2. Voltage to Current Stage

The circuit in Figure 2 converts the control voltage from the

DAC into a current that drives the LED. The MOSFET in the

circuit is able to handle currents of up to 6.3 A. However, the

current is limited to 1 A, which is the maximum rated current

of the LED.

The maximum current is limited by the resistors on each

channel: R7, R14, and R21. The maximum current can be

calculated by

IMAX = 2.048 V/R (Ω) =1.024 A

where R = 2 Ω.

Power Dissipation and Thermal Considerations

When driving LEDs with a linear current source, the power

dissipation of the sense resistors and MOSFETs must be

considered. Power dissipation in the sense resistors is always

less than their 3 W rating, even at the maximum setpoint

current of 1 A. MOSFET power dissipation increases if the LED

supply voltage is increased, resulting in more voltage drop

across the MOSFET. When used with the CFTL-LED-BAR and

a supply voltage of 16 V, keeping below half scale (500 mA)

reduces dissipation. Under these conditions, the board

temperature reaches approximately 79°C in the vicinity of the

MOSFETS with the board positioned vertically in still air. The

LTspice simulation can also be used to estimate power

dissipation for other load conditions.

For higher power levels, a heat sink or forced convective cooling

(fan) may be required to keep the board temperature lower than

130°C, the recommended maximum for FR-4. Figure 3 shows

the board operating in free air, at half of its maximum power on

Channel A at 500 mA, with an input voltage of 16 V.

The resistors reach 95°C, and the area around the FETs is

approximately 79°C.

Figure 3. Board Temperature at 500 mA

The board has as an area where there is no copper pour (shown

in Figure 4), which acts as a thermal isolation barrier from the rest

of the circuit. This area can be seen in Figure 3, where there is a

drastic drop in temperature across the two ground planes. This

barrier helps keep the temperature drift due to the components

on the left side of the board to its minimum.

Figure 4. Ground Layer—Thermal Barrier

ADA4500-2

2Ω

OUT

16405-002

16405-003

16405-004

CN-0410 Circuit Note

Rev. 0 | Page 4 of 7

Isolated Daisy-Chained SPI Control

The circuit also has a SPI communications protocol repeater,

which is enabled by the LTC6820. This device enables multiple

boards to be connected to a single twisted pair cable with only

one master controlling the whole chain.

The LTC6820 provides bidirectional SPI communications

between two isolated devices through a single twisted pair

connection. Each LTC6820 encodes logic states into signals that

are transmitted across an isolation barrier to another LTC6820.

The receiving LTC6820 decodes the transmission and drives the

slave bus to the appropriate logic states. The isolation barrier is

bridged by a simple pulse transformer to achieve basic isolation

on the CN-0410. With layout optimization, an isolation voltage

of 1500 V can be achieved, limited by the transformer. Refer to

the LTC6820 for other recommended transformers.

The LTC6820 drives differential signals using matched source

and sink currents, eliminating the requirement for a transformer

center tap and reducing electromagnetic interference (EMI).

Precision window comparators in the receiver detect the

differential signals.

The drive currents and the comparator thresholds are set by a

simple external resistor divider, allowing the system to be

optimized for required cable lengths and desired signal-to-noise

performance.

Integral Nonlinearity (INL) and Differential Nonlinearity

(DNL) Measurements

Measured results show that the combination of the AD5686,

ADR4520B, and ADA4500-2 is an excellent solution for high

accuracy, low noise performance applications. The ADA4500-2

maintains the linearity of the DAC with no crossover distortion.

INL error is the deviation in LSB of the actual DAC transfer

function from an idealized transfer function. DNL error is the

difference between an actual step size and the ideal value of 1 LSB.

This system solution provides a 16-bit resolution with ±1 LSB

DNL and INL. Figure 5 and Figure 6 show the DNL and INL

performance of the circuit.

Figure 5. Differential Nonlinearity (DNL)

Figure 6. Integral Nonlinearity (INL)

Note that the DNL and INL measurements exclude the 100

codes (approximately 4 mV) from the lower end of the range,

because the rail-to-rail output stage becomes nonlinear over

this region, as described in MT-035 Tutorial.

Noise Measurements

The targeted 0.1 Hz to 10 Hz noise for the whole board is to be

less than 24 μV p-p. The AD5686, ADA4500-2, and ADR4520B

contribute a calculated total of 9 μV p-p to the whole circuit.

The true noise of the circuit is measured by using a noise

measuring box with a gain of 10,000 and filtered between

0.1 Hz and 10 Hz.

It is advised to take the cleanest supply when taking the noise

measurements. Therefore, the ADICUP3029 was removed from

the setup, and the supply is taken from a 9 V battery.

Figure 7. Test Setup for Measuring 0.1 Hz to 10 Hz Noise with Gain of 10,000

The noise of the box itself and the circuit with the noise box

were measured, and their noises are 4.22 μV p-p and 8.38 μV p-p,

respectively. Uncorrelated, Gaussian noise sources add in a root

sum square (RSS) manner. Therefore, the noise of the CN-0410

()( )True Noise YuVp p XuVp p

Using this equation, the true noise of the circuit is calculated to

be 7.23 μV p-p.

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1.0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

55000

60000

65000

DNL ERROR (LSB)

CODE

16405-005

–0.5

0.5

1.0

1.5

2.0

2.5

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

55000

60000

65000

INL ERROR (LSB)

CODE

16405-006

±15V

POWER SUPPLY

EVAL-CN0410-ARDZ

BATTERY

OSCILLOSCOPE

VOUT

0.1Hz TO 10Hz

NOISE MEASURING

BOX

16405-007

Circuit Note CN-0410

Rev. 0 | Page 5 of 7

Board Layout Considerations

It is important to carefully consider the power supply and

ground return layout on the board. It is recommended that the

printed circuit board (PCB) have separate analog and digital

sections. If the circuit is used in a system where multiple devices

require an analog ground to digital ground connection, make

the connection at only one point. Bypass all power supply pins

with at least 0.1 μF capacitors. Place these bypass capacitors as

physically close as possible to the device, with the capacitor

ideally right up against the device. Choose the 0.1 μF capacitor

to have low effective series resistance (ESR) and low effective

series inductance (ESL), such as ceramic capacitors. This 0.1 μF

capacitor provides a low impedance path to ground for

transient currents. Use large traces for the power supply lines to

provide a low impedance supply path. Use proper layout,

grounding, and decoupling techniques to achieve optimum

performance (see the MT-031 Tutorial, Grounding Data

Converters and Solving the Mystery of AGND and DGND and

MT-101 Tutorial, Decoupling Techniques).

The AD5686 is available in 16-lead LFCSP or 16-lead TSSOP

packages. The ADR4520 is available in an 8-lead SOIC package,

and the ADA4500-2 is available in 8-lead MSOP or 8-lead

LFCSP packages.

COMMON VARIATIONS

For a lower power consumption solution (at lower speed), use

the ADA4505-1/ADA4505-2/ADA4505-4 as the output buffer.

The ADA4505 family is a micropower, zero-crossover distortion

amplifier with low input bias current.

The ADR420, ADR430, and ADR440 are suitable candidates to

provide the 2.048 V reference. These devices feature high accuracy,

low noise, and accept input voltages of up to 18 V.

For a lower power DAC, the AD5064 is a quad-channel DAC

with lower power consumption than the AD5686.

CIRCUIT EVALUATION AND TEST

This circuit uses the EVAL-CN0410-ARDZ circuit board, the

CFTL-LED-BAR, and the EVAL-ADICUP3029. The EVAL-

CN0410-ARDZ is stacked on top of the EVAL-ADICUP3029

using the Arduino-compatible pins

Equipment Needed

 PC with a USB port and Windows® 7 or above

 EVAL-CN0410-ARDZ circuit evaluation board

 CFTL-LED-BAR evaluation board

 ADICUP3029 evaluation platform or equivalent interface

 CrossCore Embedded Studios (IDE)

 Power supply: 9 V to 19.2 V wall wart

Getting Started

Load the sample code onto the CrossCore Embedded Studios

IDE by following the instructions in the Quick Start User

Guide.

Functional Block Diagram

Figure 8 shows the functional block diagram of the test setup.

Figure 8. Test Setup Functional Block Diagram

Setup

Connect the EVAL-CN0410-ARDZ by mounting it on top of

the EVAL-ADICUP3029 board using the Arduino-compatible

headers with their corresponding headers, as shown in Table 1.

Table 1. Connection Between EVAL-CN0410-ARDZ and

ADICUP3029 Boards

EVAL-CN0410-ARDZ ADICUP3029

DIGI 1 P6

DIGI 0 P7

POWER P4

ANALOG P3

Table 2. Connection Between EVAL-CN0410-ARDZ and

CFTL-LED-BAR Boards

CN-0410 Connector CFTL-LED-BAR Connector

P1.1 P1.2

P1.2 P1.1

P6.1 P5.2

P6.2 P5.1

P9.1 P9.2

P9.2 P9.1

Then connect the USB cable from the debug port of the EVAL-

ADICUP3029 to the USB port of the PC.

USB A

USB

MICRO

EVAL-ADICUP3029

EVAL-CN0410-ARDZ

RECEIVE

TRANSMIT

16405-008

CN-0410 Circuit Note

Rev. 0 | Page 6 of 7

Test

With the sample code built and loaded onto the EVAL-

ADICUP3029, and the EVAL-CN0410-ARDZ mounted on top

with the CFTL-LED-BAR connected, the device communicates

with the PC, and codes can now be written to the device via the

UART. The circuit can be tested by varying the code written to

the board to alter the intensity of the LEDs.

For complete information and details regarding test setup and

how to use the software and hardware combined, refer to the

CN-0410 User Guide.

More information regarding the EVAL-ADICUP3029 board is

available in the EVAL-ADICUP3029 User Guide.

Figure 9. EVAL-CN0410-ARDZ Evaluation System

16405-009

Circuit Note CN-0410

Rev. 0 | Page 7 of 7

LEARN MORE

Kester, Walt. The Data Conversion Handbook, Chapter 3 and

Chapter 7, Analog Devices. 2005.

MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.

Analog Devices.

MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs,

Analog Devices.

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of AGND and DGND. Analog Devices.

MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and

Rail-to-Rail Issues, Analog Devices.

MT-101 Tutorial, Decoupling Techniques, Analog Devices.

Data Sheets and Evaluation Boards

AD5686A Data Sheet

ADA4500-2 Data Sheet

ADR4520 Data Sheet

LTC6820 Data Sheet

ADP7112 Data Sheet

REVISION HISTORY

6/2018—Revision 0: Initial Version

(Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors.

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