OPTOFAST-2

Features

Opto-Isolator card
4-Channels
4N25 Optoisolators
Input /Output isolation
Transistor driver for Opto LED reduces required drive from 10 mA to under 500 uA
Input channels share their own common VCC/Ground
Output channels share their own common Vcc/Ground
8-pin header for input
8-pin header for output
Configuration Options
- Speed enhanced
- Bypass Input transistors
- Remove Output collector resistors
49x49mm
(4) 4-40 mounting holes

Schematic (One channel)

U1 represents U1-U4 (channels 1-4)
Q1 represents Q1-Q4 (channels 1-4)
R1 (VCC) represents R1,R4,R7,R10 (channels 1-4)
R2 (DIR) represents R2,R5,R8,R11 (channels 1-4)
R3 (BASE) represents R3,R6,R8,R12 (channels 1-4)
R13 (FAST) represents R13-R16 (channels 1-4)
R17 (COLL) represents R17-R20 (channels 1-4)

Input Options

Refer to schematic for impact of options

Use Input Transistor - Standard Option

Common VCC in option
Install transistor Q1
R1 = 150
- LED current is 3.5V/150 Ohms = 23 mA
- CTR = .20, 4.7 mA
R2 = Do not populate
R3 = 10K
- Base drive ~ (Vin - 0.7V)/R3
- At 5V drive this is 4.3V/R3
  - Using R3=10K at 5V this is 0.43mA Ib
  - HFE (MMBT3904) = 100 min
  - That is potentially 43 mA (in the switching range of the transistor)
    - Collector resistor and LED voltage drop are current limiter
- At 3.3V drive this is (3.3-0.7)/10K = .26mA Ib
  - HFE = 100 min
  - That is 26 mA (in the switching range of the transistor)
H1 do not short

Direct Drive - Special Option

Do not install Q1
Do not install R1
R2 = 150
- Input current = LED current = 3.5V/150 Ohms = 23 mA
Do not install R3
Do not install Q1
Short H1

Output Options

Standard Output

R17 = 1.2K
- 5V/1.2K = 4.1mA of output transistor B-E current
- Leaves 4.7-4.1 mA = 600 uA of output low current

Open Collector Output - Special Option

Do not install R17

Fast Option - Standard

R13 = 20K (empirically determined from the scope eye)

Connectors

P1 - Input

8-pin, right angle, 0.1" pitch

Vcc (In)
Vcc (In)
IN 1
IN 2
IN 3
IN 4
GND (In)
GND (In)

P2 - Output

8-pin, right angle, 0.1" pitch

Vcc (Out)
Vcc (Out)
OUT 1
OUT 2
OUT 3
OUT 4
GND (Out)
GND (Out)

Test Card Measurements

Setup

Goal was to determine the performance of the OPTOFAST card
5V operation
Same 5V input/output power supply
Each of the channels were configured uniquely/individually
MHS-5200A - Digital DDS Dual-channel Signal Generator
- 1 KHz square wave
- 5V signal level
Single channel checked at a time
Oscilloscope
- Rigol DS1054Z
- 50 MHz
- Ultra Sigma software used for screen captures

Notes

Note that the input buffer transistor inverts the signal
Built a card that has four different configurations
Options
- Fast/Slow
- Direct drive input LED/Transistor Buffered input

Test Results

Channel Fast/Slow TransBuff/Direct Input Rise/Fall Time(uS)

1 Slow Transistor Buffer Fall 28.6 1 Slow Transistor Buffer Rise 1.8 2 Fast Transistor Buffer Fall 6.68 2 Fast Transistor Buffer Rise 3.6 3 Slow Direct Input Fall 1.84 3 Slow Direct Input Rise 27.4 4 Fast Direct Input Fall 4.24 4 Fast Direct Input Rise 5.28

Observations

Input Transistor effect
- Value of the input transistor is that does not require a high drive level input
- The input transistor has very little effect on speed - a couple hundred nS slower than direct drive
- The inversion effect of the input transistor moves the "slow" time around
- Downside of the input transistor is that it has less noise immunity since the transistor turns on at 0.6V vs the internal LED in the optocoupler at 1.5V?
Output "fast" circuit
- Output pullup of 4.7K affects the rise time of the output
- The fast is faster, but not in some conditions
- Average of rise/fall is better
- Transistor buffer instance rise input is slower with the fast output
- Fast output helps the output rise time