Synchronous pulse train generation using Arduino

Using arduino, i have generated synchronized pulses with a fixed delay between them. The parameters can be remotely adjusted using serial port.

The pulse 2 (Blue) goes high after some user settable delay of Pulse 1(Yellow) rising edge.

 If we set td=tm, we can get consecutive pulses as below:

Analog signal isolation using HCNR200/201 optocoupler

 

The HCNR200/201 is a high-linearity analog optocoupler which can be used to isolate analog signals. It offers good stability, linearity, bandwidth, simple design and low cost. With choice of suitable application circuit it can also incorporate amplification, attenuation, offset, inversion among others to the input signal. 

 Details of the IC along with datasheet can be found in the link below.

https://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/high-linearity-analog/hcnr200

 

 At first glance, understanding the application circuit is difficult for a new user. 

To get complete isolation, make sure that grounds as well as bias supplies of input and output stage are isolated. Note the label used in above circuit for input stage are GND, VCC, VEE and that of output stage are GND2, VCC2 and VEE2. I generally use two isolated DC to DC converters each powering one stage at a time.

Now coming back to working of the circuit. Lets us analyze the input stage first.

The input side op-amp always tries to force same input voltages at its two input terminals in close loop connection. Thus, the input side  photodiode PD1 (terminal 3,4 of HCNR200) will have zero voltage across it. A positive voltage at inverting terminal of U1 will swing the output to negative rail causing current flow through LED (terminal 1,2 of HCNR200). Also the positive voltage will cause a current through R1 which will eventually flow through photodiode PD1.

IPD1 = Vin/ R1

Current is linearly related to input voltage. 

Since photodiode PD1 and PD2 are identical to each other, IPD2 should be equal to IPD1 ideally. Practically the relation is

 IPD2 = K x IPD1

where K is gain coefficient.

Output voltage can be given as Vout =  R2 x IPD2

Differential, Referenced single ended (RSE), Non-referenced single ended (NRSE) signals interfacing with DAQ

Before acquiring data using the DAQ board the following points must be considered

•The nature of the signal source(grounded or floating)

•The grounding configuration of the amplifier on the DAQ board

Finding a Common Ground 

Earth ground - potential of the earth below your feet.

         Most electrical outlets have a prong that connects to the earth ground which is usually wired into the building electrical system for safety. Many instruments are also” grounded” to this earth ground System ground. This type of grounding is for safety.

         Ground Symbol 

 

        Referenced Signal

        Reference ground -return path or signal common.This is usually the reference potential. The common ground may or may not be wired to the earth ground.

         Many instruments ,devices and signal sources provide a reference (the negative terminal,common terminal  etc that gives meaning to the voltages that we are measuring.

         Signal Source Reference Configuration

         Signal sources come in two forms-(1) Referenced sources called grounded signals, (2) Non referenced sources called floating signals

        Grounded signal sources have voltage signals referenced to a system ground, such as earth or a building ground.

 

        Devices that plug into the building ground through wall outlets such as signal generators and power supplies are examples of Grounded signal sources.

         Floating signal sources contain a signal that is not connected to an absolute reference such as earth or a building ground. Batteries, Thermocouple, transformers are examples of floating signal sources.

 

         Differential Connections (DIFF configuration):

        Differential connections are those in which each  analog input signal has its own reference signal or signal return path. Each input signal is tied to the positive input of the instrumentation amplifier, and its reference signal, or return, is tied to the negative input of the instrumentation amplifier.

        When configuring the DAQ for DIFF input, each signal uses two of the multiplexer inputs-one for the signal and one for its reference signal.

         Therefore, only half the analog input channels are available when using the DIFF configuration.  

         Conditions for using DIFF configuration:

         Use the DIFF input configuration when your DAQ system has any of the following conditions: 

•             Input signal levels are low (less than 1V)

•             Leads connecting the signals to DAQ system are at long distance.

•             Any of the input signals require a separate ground-reference point or signal-return.
  

•             The signal travels through a noisy environment.

•  

A differential instrument requires two inputs where neither input to the instrumentation amplifier is referenced to a system ground. For example, CH0+ and CH0- are wired into the positive and negative terminals of the instrumentation amplifier respectively, but they are not connected to the measurement system ground (AI GND).

       The differential voltage across the circuit pair is the desired signal, yet an unwanted signal that is common to both sides of a differential circuit pair can exist. This voltage is known as common-mode voltage. An ideal differential measurement system completely rejects, instead of measures, the common-mode voltage for more accurate measurements. Practical devices, however, have limitations described by specifications such as common-mode voltage range and common-mode rejection ratio (CMRR).

          Differential input connection for Grounded sources

 

Differential input connection for floating sources

 

 

RSE and NRSE connections

     RSE measurement system is used to measure a floating signal, because it grounds the signal with         respect to building ground.

 

     NRSE measurement system, all measurements are made with respect to a common reference, because all of the input signals are already grounded.

     Use the RSE configuration for floating signal sources; in this case, the DAQ boards provide the     reference ground point for the external signal.  

        Use the NRSE configuration for ground-referenced signal sources; in this case, the external signal     supplies its own reference ground point and the DAQ boards should not supply one.