Psychology works

Post by RC, Sep 30, 2016.
Psychology Mistake

I was so confident that I have ordered correct parts for my project that I went ahead and placed an order for 100 units ($9 each).

Discharging a capacitor with constant current

Post by RC, Sep 28, 2016.
Capacitor Discharge

Sleep mode current is under 20uA - great. Everything was working just fine until we tried to reset the unit by disconnecting all the power.
Since unit consumes only 18uA in the sleep mode it takes forever for the unit to reset. The reason is the total amount of capacitance on my
board. I have estimated that the total capacitance on the board is about 740 μF.

Testing AC powered equipment

Post by Me, Sep 23, 2016.
Test Equipment

Oscilloscopes are used to observe the change of an electrical signal over time, such that voltage. AC powered oscilloscopes are very common in engineering labs. Oscilloscope is a first aid help test and measurement device when a signal waveform need to be analyzed. Taking reads from an AC powered equipment with oscilloscopes may create a low impedance short between an o-scope AC ground and equipment that is being tested. The ground clip on a o-scope is actually tied to the ground. In order to avoid this problem differential probes can be used, but they are costly. Multimeter + has two differential inputs that have 10 M-Ohm input impedance. Both of these inputs can be used to view wave-forms up to 1 kHz (-3dB). High input impedance guarantees that there will be no shorts introduces when taking measurements from an AC powered equipment.

Differential input with overvoltage protection of the Multimeter+:

The USB port of the Multimeter + is electrically isolated from a PC with thehelp of ISO7221 Dual Channel Digital Isolator.

These devices have a logic input and output buffer separated by TI’s silicon-dioxide (SiO2) isolation barrier, providing galvanic isolation of up to 4000 V. Used in conjunction with isolated power supplies, these devices block high voltage, isolate grounds, and prevent noise currents on a data bus or other circuits from entering the local ground and interfering with or damaging sensitive circuitry. Isolated USB ensures that there will be no interference introduced by a PC power supply. The ISO72x family of products consists of an input and an output semiconductor device separated by a high-impedance isolation barrier designed for the transfer of electrical signals across the barrier. The ISO72x uses capacitive coupling to transmit signals across the barrier while maintaining isolation of the output side with reference to input. The capacitor dielectric is semiconductor-grade silicon dioxide and is the isolation barrier. Figure 1 shows the capacitor is constructed from a top plate consisting of plated copper and a bottom plate made from doped silicon substrate. A BCB (benzocyclobutene) spin-on dielectric passivation over the top plate reinforces the insulation characteristics. (TI's application note: "High-Voltage Lifetime of the ISO72x Family of Digital Isolators" )

Typical application schematics:

The input and output signals are separated by an isolation barrier.

Datasheet: ISO7221

Now that I have my Multimeter+, I have tested ISO7221 with the USB connection. Unfortunately it does not isolate 100%. There is a small amout of DC leakage introduced by a PC ground loop. Basically voltage reading shift by about 0.5mV when Multimeter+ is connected to a PC. To fix the issue I disconnect my laptop from AC lines and run it on the battery.

Data Representation in the Multimeter+

Post by Me, Sep 23, 2016.
Test Multimeter+

Data is stored and processed in a fixed-point representation. In a fixed-point processor, numbers are represented and manipulated in integer format. In a floating-point representation numbers are represented by the combination of a mantissa (or a fractional part) and an exponent part. In a fixed-point processor, one needs to be concerned with the dynamic range of numbers, since a much narrower range of numbers can be represented in integer format as compared to floating-point format.

Q-format Number Representation The decimal value of an N-bit 2’s-complement number, B = bN-1bN-2…b1b0, where bi is either 0 or 1, is given by

The 2’s-complement representation allows a processor to perform integer addition and subtraction by using the same hardware. When using the unsigned integer representation, the sign bit is treated as an extra bit. Since my device have to be able to measure positive, negative, as well as AC signals the numbers are represented in sign magnitude format. This allows me to keep track of AC signal for automatic switching between DC and AC measurements. Converting 2’s complement values to sign magnitude requires processing time, but with 16 MIPS speed (PIC24FJ256GA110) it is unnoticeable.

There is a limitation of the dynamic range. For the 24 bit data (MCP3911) maximum positive value is +8388607 and max negative value is -8388608. These values represent number of steps in positive and negative directions. Measured values are represented in Nano-Volts and Micro-Volts, this allows me to keep high resolution of a measurement. To display measurements in Volts I move decimal point accordingly.
This representation scheme is referred to as the Q-format or fractional representation.

bN-1 2N-1. bN-2 2N-2 … b1 21 b0 20 Volts or Amps.

The binary number gets converted to a string of characters to be displayed.

Simple Additive Mixer

Post by Me, Sep 26, 2016.
Additive Mixer

Simple additive mixers use Kirchhoff's circuit laws to add the currents of two or more signals together. When two different signals are applied to an input of an amplifier (assuming the amplifier input impedance is much higher than the source impedances of the two signal generators) they will add At Vin Junction point.

E-ink Displays

Post by Me, Sep 23, 2016.
E-ink Displays

E-Ink displays are Electrophoretic (which is motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field), they are made of tiny capsules (0.04mm in diameter) that contain two kinds of particles: black and white. Using electricity one can choose whether the white or black particles will rise to the top of the capsule - and so change the color of the pixel. Those particles remain in place when no electricity is used - and so the displays do not need power when the image does not change.