• Revision 2

    michal777 • 01/01/2025 at 13:00 • 0 comments

    Revision 2 is made on 4 layers board and has some shielding to improve noise immunity. Low frequency noise immunity is good but immunity to high frequency noise is still not perfect. Photodiodes were initially not shielded so I had to wrap them in a copper foil to improve immunity. There's still some work to do, to minimize the susceptibility - it's not easy, because the signal is in milivolts range and the device may work near noisy equipment. Possibly a different construction, with very good shielding, including photodiodes, will be needed.

    In this revision OPV332 VCSEL emitters were used. Unfortunately they are not perfect - they are usable in small range of power, but with higher power the noise and non-linearity is too bad (the nonlinearity seems "unstable", sawtooth is "wavy" and its shape is easily changed by touching the optical assembly or by small changes of input signal). I don't know reasons of the weird nonlinearity (is it only VCSEL property, or it's more a problem of interaction with photodiode?).

    Possibly two versions can be considered in future:
    - variant with VSLY3943 LED emitters - linearity is good, bandwidth limited below 10MHz

    - variant with OPV332 VCSEL emitters - bandwidth over 30MHz, worse linearity and noise



    Issue

    Status

    Resonance of input wires

    A lot of ringing is visible at fast edges. This could be expected and can be reproduced in simulation. Inductance of wires for signal input (hundreds of nH) resonates with input capacitors (about 5pF).

    Add 470R resistor in series with input to prevent resonance of the wires with the input capacitance. This 470R is small enough to not cut usable bandwidth.

    Bias of TLV431 for laser overload protection

    The TLV431 connected directly to the non‑inverting input of the op‑amp changed operating point so that it doesn’t match the secondary side op‑amp.

    The TLV431 should have cathode pulled up to positive voltage and be connected through diode to the non‑inverting input of the op‑amp (the diode is not conducting during normal operation).

    Voltage drop on photodiode bias

    There’s a noticeable voltage drop on a 1k resistor at the output of power supply bias output.

    Change the resistor to 100R.

    (maybe consider replacing with ferrite, also test if larger filtering capacitor is needed).

    No clean turn-off when battery is low

    Slow falling edge at the output of the MOSFET doesn’t turn off TLV431 immediately, also battery voltage increases when MOSFET turns off

    To be done: try tuning, maybe add more transistors, detect falling edge, further complicate the design

    Distortion at some voltage level

    Weird distortions observed when OPV331 laser diode was used. Decreasing photodiode current (by decreasing laser current or blocking some light) makes the distortions lower (it also lowers noise).

    Maybe two versions are to be considered:

    - OPV331 – high speed, worse linearity, bias optimized to lower distortions and noise (low photodiode current, low input amplitude)

    - VSLY3943 (alternatively VSMY5850 +external lens*) – low speed, high linearity, larger overshoot because of delay in feedback loop (compensating capacitor can be selected to decrease overshoot or increase bandwidth)

    - VSLY3943 – R27=1k6, R39=0R, R37=100R, C41=2p2 (To be done: optimize R37, R39, maybe R21**)

    - OPV331 – R27=?, R39=120R, R37=100R, C41=NA (To be done: optimize R27, test different R21, maybe change input divider or range)

    * VSMY5850 has lower wavelength (should be good for response time) but nominal response time is lower, it’s SMD and external lens may be needed, possibly slightly better (more tests needed…)

    ** If emitter is slow, R21 could be increased (slower detector might be not an issue) – tested change from 470R to...

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  • Revision 1

    michal777 • 01/01/2025 at 12:27 • 0 comments

    The primary side consists of: power supply, input signal divider (1/10, 1/100, 1/1000), low input capacitance buffer and operational amplifier that drives IR emitters. Feedback of the amplifier includes additional IR emitter and photodiode (the same as on the secondary side) - it's needed for linearization of characteristics of the emitter and detector.

    The secondary side consists of: power supply and amplifier that converts photodiode current to the output voltage.

    If emitters and detectors have similar characteristics (preferably they are paired), then voltage on R21 and (R42+R46) should be identical (good linearity).

    The main problems with the first revision was susceptibility to electromagnetic disturbances: switching noise from internal converters and external SMPSs, also picking up low frequency noise by input voltage divider. Also there were some issues with power supplies or other design issues.

    Initially, the infrared emitter was LED, then I started testing VCSEL. VCSEL provided more power at lower current and it was much faster (about 40MHz achieved with VCSEL, with LED it was only about 10MHz). The used VCSEL could be easily overloaded so some active protection was needed in next revision.


    Issue

    Status

    Can’t turn on.

    Under-voltage battery protection issue. Q1 doesn’t turn on after closing the switch. It can turn on if C1 is increased from 1u to 10u. R13 can be increased too.

    C1 and resistances are increased. Larger time constant is needed because it takes time to start the boost converter.

    Power not delivered at low battery voltage.

    Under-voltage battery protection issue. Q1 doesn’t fully turn on if input voltage is low (about 3.4V). Voltage is too low for TL431 so it’s unable to properly bias gate of Q1.

    TL431 (Vref=2.495V) replaced by TLV431AFTA (Vref=1.24V). The same reference will be used in other places of the circuit. New resistors values for TLV431AFTA were selected.

    Unstable step-up converter.

    As datasheet suggests, low inductance and high output capacitance is good for stability. L1 and L2 15uH, C2 and C51 3x10uF works OK.

    Tested with #A920CY-150M=P3 (15uH) and output capacitors 3x10u works well. A sililar SRR5028-150Ycan be used instead. The used inductors are magnetically shielded to minimize noise.

    Noisy power supply.

    Resistors were added at inputs of charge pumps (R15, R20, R40) but they are not enough. Replacing then with ferrite beads helps (1k@100MHz used but it’s maybe too much).

    Added ferrite beads at inputs of charge pumps 600R@100MHz.

    No signal - saturation.

    Incorrect bias for the used emitter+photodiode (VSMY5850 and SFH203). R26 and R44 had to be much lower because VSMY5850 can’t cause that much current through SFH203.

    VSMY5850 won’t be used, bias should be designed for IR emitter OPV332. Potentiometers were added to make it adjustable (OPV332+SFH203 may be not very repeatable). Tests shown that 0.7V at “+” input is good for range and linearity (so about 1.5mA through photodiode should flow at 0V input) – voltage divider was adjusted.

    Could be faster.

    Emitter VSMY5850 and photodiode SFH203 were good up to about 10MHz, C41 was required to avoid ringing. OPV332 (VCSEL) instead of VSMY5850 (LED) works better:

    - higher signal (0.4Vpp at output),

    - wider bandwidth (probably 30MHz),

    - lower current - less heating, so better thermal stability expected (10mA is max, now limited to 5mA).

    - THT package (not SMD-THT adapter required)

    Higher cost but it’s worth the money.

    R30 is changed from 33R to 220R. C41 not placed.

    VSMY5850 was changed to OPV332. Cathode must be connected to -5V (not GND) to ensure voltage sufficient for bias (considering op-amp output range and max voltage drop at OPV332). Op-amp output...

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