SainSmart DDS120

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Revision as of 00:17, 26 March 2016 by Merbanan (talk | contribs) (→‎Gain stage: elaborate the reg values for the gain step)
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SainSmart DDS120

The SainSmart DDS120 is a USB-based, 2-channel oscilloscope with an analog bandwidth of 20MS/s and 48MS/s sampling rate.

See SainSmart DDS120/Info for more details (such as lsusb -v output) about the device.

or in newer hardware

  • AD: MXT2088 (datasheet)
  • Opamp: Analog devices AD8065 (OPAMP)
  • MUX: Texas Instruments CD4052B (2 x 1:4 MUX)

Firmware

The Sainsmart firmware is contained on an i2c eeprom. The FX2 boots directly into that firmware. Loading another firmware is possible but the os needs to support soft reset so it can reload the usb descriptors (linux does so via the USBDEVFS_RESET ioctl).

Vendor Protocol

Oscilloscope command bRequest value Notes
Set CH0 coupling 0x24 Possible values: 0x00, 0x08 (AC-coupling, DC-coupling).
Set CH1 coupling 0x25 Possible values: 0x20, 0x10 (AC-coupling, DC-coupling).
Set CH0 voltage range 0x22 Possible values: 0x08, 0x04, 0x00, 0x06, 0x02 (50mV, 100mV, 200mV, 500mV, 1-5V).
Set CH1 voltage range 0x23 Possible values: 0x20, 0x10, 0x00, 0x12, 0x02 (50mV, 100mV, 200mV, 500mV, 1-5V).
Set sampling rate 0x94 Possible values: 0x11, 0x01, 0x10 (240kHz, 2.4MHz, 48MHz).
Trigger oscilloscope 0x33 Possible values: 0x00 == start sampling.

Photos


Resources

Gain stage

The gain stage is 2 stage approach. -6dB and -20dB on the first stage (attentuator). The second stage is then doing the gain by 3 different resistor values switched into the feedback loop.

#Channel 0:
PC1=1; PC2=0; PC3= 0 -> Gain x0.1 = -20dB -> 0x02 (1V)
PC1=1; PC2=0; PC3= 1 -> Gain x0.2 = -14dB -> 0x0A (0.5V)
PC1=1; PC2=1; PC3= 0 -> Gain x0.4 =  -8dB -> 0x06 (0.250V)
PC1=0; PC2=0; PC3= 0 -> Gain x0.5 =  -6dB -> 0x00 (0.200V)
PC1=0; PC2=0; PC3= 1 -> Gain x1   =   0dB -> 0x08 (0.100V)
PC1=0; PC2=1; PC3= 0 -> Gain x2   =  +6dB -> 0x04 (0.050V)
#Channel 1:
PE1=1; PC4=0; PC5= 0 -> Gain x0.1 = -20dB -> 0x02 (1V)
PE1=1; PC4=0; PC5= 1 -> Gain x0.2 = -14dB -> 0x22 (0.5V)
PE1=1; PC4=1; PC5= 0 -> Gain x0.4 =  -8dB -> 0x12 (0.250V)
PE1=0; PC4=0; PC5= 0 -> Gain x0.5 =  -6dB -> 0x00 (0.200V)
PE1=0; PC4=0; PC5= 1 -> Gain x1   =   0dB -> 0x20 (0.100V)
PE1=0; PC4=1; PC5= 0 -> Gain x2   =  +6dB -> 0x10 (0.050V)

AC/DC coupling

The coupling is controlled via 2 PhotoMOS chips.

#Channel 0:
PE3=0 Disable AC-coupling capacitor (DC-Coupling)
PE3=1 Enable AC-coupling capacitor
#Channel 1:
PE0=0 Disable AC-coupling capacitor (DC-Coupling)
PE0=1 Enable AC-coupling capacitor

1kHz Calibration square wave

Pin PE2 controlls the level of the calibration output on the pcb. This is most likely driven by an ISR timer from the 8051 core of the FX2 chip.