Common Analog Functions Using an iCE40 FPGA

TM
Common Analog Functions
Using an iCE40 FPGA
Integrate discrete analog devices into the iCE40 Ultra-Low Density FPGA
Analog Functions in an FPGA
FPGA designers are familiar with implementing digital designs by
using FPGAs to glue together various processors, memories, and
standard functions. In addition to these digital functions, FPGAs
can also be used to implement some common analog functions.
By using an LVDS input, a simple resistor capacitor (RC) circuit
and some FPGA digital logic elements, designers can create a
comparator, Analog-to-Digital Converter (ADC), Digital-to-Analog
Converter (DAC), or simple temperature monitoring system.
VcmH
Vcm
VcmL
OUT
IN_N
VCCIO=1.8V
VCCIO=1.8V
Units
Common mode voltage high
VCCIO/2 + 0.2
VCCIO/2+ 0.3
V
Vcm
Common mode voltage
VCCIO/2
VCCIO/2
V
VcmL
Common mode voltage Low
VCCIO/2 - 0.2
VCCIO/2-0.3
V
Icomp
ICCIO per comparator
160
160
µA
generated by photo diode current, IN_P. When infrared light is
applied, OUT will switch from low to high.
The iCE40™ FPGA family offers 2 to 23 Low Voltage Differential
Signal (LVDS) I/O pairs depending on density and package
combinations. These differential I/O pairs are ideal for implementing
common analog functions such as comparators, ADC and DAC.
Figure 1 and Table 1 show the LVDS buffer and the comparator’s
common mode range. Input signals IN_P and IN_N are compared
and the result is output to signal OUT as shown in Figure 2a. When
the voltage of one of the inputs is out of the common mode range
between VcmL and VcmH, then the other input must remain within
the common mode range between VcmL and VcmH. Operating
current adds 160 µA per comparator to ICCIO. An example
application of the comparator is an infrared receiver is shown in
Figure 2b. Reference voltage IN_N is compared with the voltage
IN_P
Description
VcmH
Table 1: Common mode operating ranges
Basic Comparator
VCCIO
Symbol
Within the specified common mode range described in Table 1,
input signals IN_P and IN_N are compared and the result is output
to signal OUT. For high-speed operation, both inputs must remain
within the common mode range between VcmL and VcmH. An
example application of high-speed operation is in the camera serial
interface in MIPI CSI-1. Common mode output levels from iCE40
bank 3 are set by 220Ω and 120Ω resistors at the transmitter.
100Ω resistors terminate the receiver side at the iCE40 bank
3 comparator inputs, setting the high voltage level at 3/5 * 1.8 =
1.08 V and the low voltage level at 2/5 * 1.8 = 0.72 V, within the
specified common mode range.
OUT
IN_P
IN_N
IN_P
IN_N
+
-
OUT
GND
Figure 1: Comparator in iCE40
Figure 2a: Comparator waveform
Figure 2b: Infrared receiver example
iCE40 FPGA
Camera
RGB Tx
220Ω
220Ω
SERDES
iCE40 FPGA
Clock lane
120Ω
100Ω
LVDS
LVDS
220Ω
220Ω
120Ω
SERDES
Receiver
Application
Processor
100Ω
Data lane
Figure 3: A comparator in the iCE40 MIPI CSI-1 application
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Digital-to-Analog Converter (DAC)
A Delta-Sigma DAC can be implemented using an iCE40 FPGA.
Figure 4 shows a block diagram implementing a digital-to-analog
converter using the iCE40. A first, second or third order Delta-
Sigma modulator can be implemented for desired performance. It
offers 8 to 10-bit resolution with a 44 KHz sampling rate and is
ideal for audio related applications.
iCE40 FPGA
MP3
Data
Flash
PCM
Interpolator
Analog Output
Delta-Sigma
Modulator
Low Pass Filter
Head Phone /
Audio Amplifier
Figure 4: Implementing an Audio Decoder and Digital-to-Analog Converter with an iCE40 FPGA
Analog-to-Digital Converter (ADC)
Similar to a DAC, an analog-to-digital converter can be
implemented using an iCE40 FPGA. A first order Delta-Sigma
ADC with 8-bit resolution and a sampling frequency of 8KHz
is implemented inside an iCE40 FPGA. This is ideal for sensor
applications such as pressure, temperature, voltage and
acceleration or motor control. Figure 5 provides a block diagram of
an ADC in an iCE40 FPGA.
iCE40 FPGA
Feedback Resistor
Analog
Input
Stable
Reference
Voltage
+
D
Q
C
Q’
CIC
Filter
Decimation
Filter
Digital
Output
-
Figure 5: Implementing an Analog-to-Digital Converter with iCE40
Applications Support
1-800-LATTICE (528-8423)
503-268-8001
[email protected]
Copyright © 2012 Lattice Semiconductor Corporation. Lattice Semiconductor, L (stylized) Lattice Semiconductor Corp., and Lattice (design), iCE40 and iCEcube2 are either registered trademarks or trademarks
of Lattice Semiconductor Corporation in the United States and/or other countries. Other product names used in this publication are for identification purposes only and may be trademarks of their respective
companies.
October 2012
Order #: SB012
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