AN2199
Analog - DAC with Analog Modulator
Author: M. Ganesh Raaja
Associated Project: Yes
Associated Part Family: CY8C22xxx, CY8C24xxx, CY8C27xxx, CY8C29xxx
Software Version: PSoC ® Designer™ 5.4
Related Application Notes: None
AN2199 explains the construction of a high resolution DAC using a 16-bit PWM, a switched capacitor block, and the
analog modulator function.
Introduction
PSoC® Designer already has DAC6, DAC8 and DAC9
User Modules. Any resolution beyond this is not possible
using the existing topology. This Application Note
discusses a method of using a 16-bit PWM, an SCBLOCK,
and the analog modulator function to construct a higher
resolution DAC. The example project has a 12-bit DAC
incorporated. Theoretically, a 16 bit DAC can be designed
using this technique. But the repeatability and effect of
noise have to be studied for such high resolutions.
Type C Switched Capacitor blocks in ASC10, ASC21,
ASC12, and ASC23 all have the function of analog
modulation. There are eight possible signals that can be
used to modulate the analog signal. These include the
analog comparator bus outputs, two global outputs, and a
digital block broadcast bus. The selection is done by
modifying the bits in the AMD_CR0 and AMD_CR1
registers. The modulation signal is XOR’ed with the Sign
bit of the switched capacitor block to determine the sign of
the output.
Configuration
Analog Modulator
Figure 1 shows the modulation control section of a Type C
Switched Capacitor block.
Figure 1. Modulation Control of a Switched Capacitor
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The user module placement and parameters are shown in
Figure 2, Figure 3 and Figure 4.
A PWM16 User Module is placed. The output of the
PWM16 is routed to the ROW_0 Broadcast Bus and the
Global_Out_Even1 Bus after inversion using the LUT. The
clock of the PWM is selected as SysClk direct. The period
of the PWM is set to 4095. So the output frequency of the
PWM is 5.86 kHz. The AMD_CRx register is updated to
select the ROW_0 Broadcast line as the Analog
Modulator. By varying the pulse width, the output voltage
of the SCBLOCK can be controlled. As the PWM output
level changes between 0 and 1, the output of the
SCBLOCK switches between REFLO (Gain of -1) and
REFHI (Gain of +1). A simple RC filter at the output of the
SCBLOCK smoothes the modulated output to a stable DC
voltage.
Document No. 001-34561 Rev. *C
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Analog - DAC with Analog Modulator
Figure 2. Digital Block Placement
The SCBLOCK User Module is placed in one of the type C blocks.
ACMux is selected as REFHI. ASign is Positive. FCap and ACap are set to 16 to set the gain to 1.
Figure 3. Analog Block Placement
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Figure 4. SCBLOCK Parameters
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Analog - DAC with Analog Modulator
The Program
Higher Resolutions
Apart from the starting of the user modules, there is no
CPU overhead involved. To update the DAC output, just
change the pulse width value of the DAC_PWM User
Module.
Higher resolutions can be achieved by changing the
period of the PWM. By changing the period to 16384, you
can construct a 14-bit DAC. But the output frequency of
the PWM will now be 1.46KHz and the RC constant used
to filter the output should be higher. To keep the output
frequency high, the input of the PWM can be selected as
SysClk*2.
void main(void)
{
Theoretically, with a 16-bit PWM, you can go up to a 16-bit
DAC. But repeatability and noise issues should be studied.
// Start the Modulating PWM
DAC_PWM_Start();
Other Applications
// Start the SC Block
DAC_Modulator_Start
(DAC_Modulator_HIGHPOWER);
Apart from using this setup as a DAC, you can also create
an MDAC with a higher resolution. For this, the ACMux
input should be routed to the external signal to be
multiplied, instead of REFHI.
// Turn On Analog Modulator and
// select Global_Out_Even_1 bus
// as modulator source
AMD_CR0 = 0x01;
Summary
The rich set of analog switched capacitor blocks of
PSoC 1 enables wide variety of options to implement a
particular function. One such function was demonstrated
in this application note to implement a 12-bit DAC using
the modulation control of a switched capacitor block.
// Start the AGND RefMux
AGND_Start(AGND_HIGHPOWER);
// Duty cycle as 25%
iPulseWidth = 1024;
// Update the Pulsewidth to change
// the DAC output
DAC_PWM_WritePulseWidth(iPulseWidth);
while(1)
{
;
}
About the Author
Name:
M. Ganesh Raaja.
Title:
Applications Engineer Principal
Background:
M. Ganesh Raaja completed his
Diploma
in
Electronics
and
Communications Engineering in 1992.
From servicing floppy drives and
printers, designing emergency lamps,
UPS, industrial transducers and
industrial automation products to
designing with PSoC, he has 17 years
of experience; 7 years of which are in
PSoC. He was a CYPro consultant
from 2002 and joined Cypress as a
Principal Applications Engineer in
September 2008.
}
Effect of Offset and Gain and its Solution
Naturally, there will be error on the DAC output due to the
offset and gain error of the SCBLOCK. This can easily be
overcome by using offset and gain correction methods.
One such method is explained in detail in Application Note
AN2117 – Analog – DAC11. This involves calculating the
DAC value for 0.00V output (offset zero) and the DAC
value for another known output, say 1.000V (full scale).
From the difference of full scale and span, the counts/volt
calibration constant can be calculated. To get any other
voltage, multiply the desired output with the counts/volt
constant, add the offset zero, and update the PWM with
this value to get the exact desired output.
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Document No. 001-34561 Rev. *C
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Analog - DAC with Analog Modulator
Document History
Document Title: Analog - DAC with Analog Modulator - AN2199
Document Number: 001-34561
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
1505943
JVY
09/26/2007
Re-catalogued application note.
*A
3211722
PMAD /
GRAA
03/31/2011
Upgraded and tested the project to PSoC Designer 5.1 SP1
*B
4339865
RJVB
04/10/2014
Updated to new template.
Completing Sunset Review.
*C
4622198
ASRI
01/13/2015
®
Updated Software Version as “PSoC Designer™ 5.4”.
Updated attached associated project to PSoC Designer 5.4.
www.cypress.com
Document No. 001-34561 Rev. *C
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Analog - DAC with Analog Modulator
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