AN-1212: Single Supply Low Noise LED Current Source Driver Using a Current Output DAC in the Reverse Mode (Rev. A) PDF

AN-1212
APPLICATION NOTE
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Single Supply Low Noise LED Current Source Driver Using a Current
Output DAC in the Reverse Mode
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
This circuit provides a low noise, single supply current drive for
an LED. Each component is selected to operate from a 3.0 V
single supply while maintaining very low peak-to-peak noise.
The signal chain is optimized for low power, low noise optical
communications and medical applications.
Table 1. Devices Connected/Referenced
Product
AD5452
AD8655
ADR127
In a typical pulse oximetry application, an LED is pulsed from a
high level of current (e.g., 3/4 scale) to a lower level of current
(e.g., 1/4 scale). The on time of these pulses is typically in the
order of several hundred microseconds. Peak-to-peak 1/f noise
superimposed on the LED brightness levels during the on time
affects the accuracy of the overall measurement. The R-2R core
of a current output DAC has inherently low 0.1 Hz to 10 Hz
noise because it is only the resistive noise of the ladder that
causes the noise. The AD5452 current output DAC is used in
the reverse mode so it can support single supply applications.
By applying 1.25 V to the IOUT pin, a full-scale code will result
in 1.25 V − 1 LSB appearing on the VREF pin, while a zero-scale
code will result in 0 V on the VREF pin.
Description
12-bit multiplying DAC
Low noise precision CMOS amplifier
Low noise 1.25 V precision LDO
The signal chain in Figure 1 shows the AD5452 current output
DAC in reverse (voltage switching) mode controlling the
brightness of an LED. By using the current output DAC in
reverse mode, described in more detail in the AD5452 data
sheet, the part can take a positive low noise 1.25 V reference
input and output a positive 1.25 V – 1 LSB output at full-scale. It
is important to note that with this configuration, the current
output DAC is limited to low input voltages (<1.5 V) because
the switches in the DAC ladder do not have the same source-todrain drive voltage. As a result, their on-resistance differs, which
degrades the integral linearity of the DAC. The ADR127 is a low
noise 1.25 V output precision LDO and provides this low voltage.
The ADR127’s high output current and low quiescent current
make it ideal for handheld medical applications.
Key to maintaining low noise in the signal chain is the ADR127
reference, which has a 0.1 Hz to 10 Hz noise of only 9 µV p-p.
In addition, the AD8655 is the industry’s lowest noise precision
CMOS amplifier (1.23 µV p-p). The combined circuit has a
typical 0.1 Hz to 10 Hz noise of only 14.7 µV p-p.
3.0V
3.0V
3.0V
RFB
ADR127
VIN
1.25V
VOUT
GND
IOUT
VDD
AD5452
3.0V
HAMAMATSU
L5766
VREF
0.1µF
AD8655
2N3904
GND
SERIAL INTERFACE
3.0V
A
0.1µF
R1 = 22.1Ω
08702-001
10µF
Figure 1. Low Noise LED Driver (Simplified Schematic: All Connections Not Shown)
Rev. A | Page 1 of 2
AN-1212
Application Note
The 12-bit AD5452 DAC generates the voltage that drives the
noninverting input of the operational amplifier. This voltage
also appears across the R1 resistor, which should be a high
precision resistor, and generates the current required at the
collector of the bipolar transistor. Nominal diode current at
full-scale is 56.6 mA for the circuit shown in Figure 1. A low
noise AD8655 is used to sense the current through R1.
The R-2R architecture of the AD5452 allows for a very low
noise DAC core. The dominating noise source in the signal
chain is the ADR127 reference with a specified typical 1/f noise
of 9 μV p-p. The plot in Figure 2 is the 0.1 Hz to 10 Hz voltage
noise at the negative terminal of the AD8655, removing the
noise added by the LED, in this case a Hamamatsu L5766. A
standard 2N3904 NPN bipolar transistor was used in the circuit
to drive the LED.
COMMON VARIATIONS
Other suitable DACs are the 16-bit single channel AD5543 or
the 14-bit AD5446. Dual channel DACs will also work, such as
the AD5447 (12-bit) and AD5545 (16-bit). The AD8656 is a
dual channel version of the AD8655.
Alternative low noise precision op amps such as the
ADA4841-1 and the ADA4841-2 are also suitable.
LEARN MORE
Kester, Walt. The Data Conversion Handbook. Chapter 3, 7.
Analog Devices. 2005.
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs.
Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
5µV/DIV
Voltage Reference Wizard Design Tool. Analog Devices
Data Sheets and Evaluation Boards
4
AD5452 Data Sheet
AD5452 Evaluation Board
14.7µV p-p
2.42µV rms
08702-002
AD8655 Data Sheet
1s/DIV
Figure 2. 0.1 Hz to 10 Hz Noise Measured at Point A
ADR127 Data Sheet
REVISION HISTORY
4/12—Rev. 0 to Rev. A
Changed Document Title from CN-0139 to
AN-1212 .............................................................................. Universal
12/09—Revision 0: Initial Version
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08702-0-4/13(A)
Rev. A | Page 2 of 2
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