Modulation Plugin

ToDos

• make sure each components freq. response, to make sure the highest freq.
• place DAC into this module board together, then only 4 wires(Vcc, I2C clk, Gnd & I2C data) from host.
• understand all components
• call sample
• draw circuit and PCB and grounding problem
• test by real circuit and firmware

Objectives

• The modulation module is used to deliver a digital output signal with the following characteristics:
  • Square wave
  • Variable frequency (>1kHz)
  • Variable amplitude (0V - 2.5V)
• interface to MCU which do the control

Q: Is square wave where the "Lo" time is always at "GND" OK? Q: If not, is a switch between "Lo is GND" vs. "Lo is negative Hi" OK? Or does "Lo" need to be independently controlled with another ADC?

Q: Will people be connecting a speaker directly to the output, requiring a relatively high-power output op amp such as the LM386N-3 ?

Software Solution

• Timer Interrupt controlling I2C 10-bit DAC can only achieve frequency response of about 1kHz
  • 5byte/cmd
  • 8bit/byte
  • baudrate = 400kHz
  • This yields 10kHz (0.1ms)
  • As OS context switch is 10ms, set timer to 1ms (1kHz), so that 0.9ms can be used to process other tasks
• Due to higher freq. response requirement, so using hardware to implement the clock

Hardware Solution

Circuit

                    +-----+            +--------+         +--------+
      I2C Bus       |     |            | Analog |         | Op-Amp |
  ------------+-----| DAC |------------| Switch |---------|        |---------Output
              |     |     |            +--------+         +--------+
              |     +-----+                |
              |                            |
              |     +--------------+       |
              |     | Programmable |       |
              +-----| Clock        |-------+
                    +--------------+
  

This architecture allows the square wave to run at a very high frequency, even with a slow I2C and a slow DAC.

Circuit Simulation by Paul Falstad's Web

• Import the following

$ 1 5.0E-6 16.817414165184545 64 5.0 50
w 128 256 144 256 0
w 16 256 64 256 0
g 16 352 16 368 0
v 64 64 112 64 0 0 40.0 2.5 0.0 0.0 0.5
w 112 128 144 128 0
w 64 128 16 128 0
w 16 128 16 256 0
v 64 256 128 256 0 2 1000.0 1.65 1.65 0.0 0.5
a 336 144 464 144 1 3.3 0.0
w 336 160 336 192 0
w 464 192 464 144 0
w 464 144 512 144 0
r 592 144 592 256 0 1000.0
g 592 352 592 368 0
w 512 144 592 144 0
w 336 192 352 192 0
w 432 192 464 192 0
v 64 128 112 128 0 1 1.0 1.25 1.25 0.0 0.5
w 16 256 16 288 0
w 16 288 16 352 0
w 592 256 592 352 0
g 272 352 272 368 0
w 272 128 336 128 0
w 176 256 176 144 0
r 272 224 272 288 0 1000.0
w 272 128 272 224 0
w 272 288 272 352 0
159 144 128 208 128 0
r 208 128 256 128 0 1000.0
w 256 128 272 128 0
w 144 256 176 256 0
r 336 224 336 288 0 1000.0
g 336 352 336 368 0
w 336 288 336 352 0
w 336 192 336 224 0
r 352 192 432 192 0 1000.0
o 4 64 0 34 2.5 7.8125E-4 0 -1
o 0 64 0 35 5.0 9.765625E-5 1 -1
o 11 64 0 34 2.5 0.0015625 2 -1

Components Inside Circuit

DAC chip

• datasheet -- http://focus.ti.com/lit/ds/symlink/dac6574.pdf

clk chip

• datasheet -- http://focus.ti.com/docs/prod/folders/print/cdc706.html
• call sample first, later to buy at -- http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=296-21381-5-ND

analog switch

• http://focus.ti.com/docs/prod/folders/print/ts5a1066.html
• call sample

op-amp buffer

• OPA340