Flow Bench Analog Input Schematic Review

Other Hardware Sensors
1

-Background-
LTC2400 Signal/Voltage Creep?

-Arduino-
Arduino Duemilanove atmega 328
USB power & Windows 7 laptop
External power from 9v battery (eventually changed to ac adapter 9-12v)
Arduino IDE 1.0.6

-Board-
-Components-
3x Yageo 1/2 watt 10k ohm resistors (through hole)
2x Panasonic 10uf electrolytic capacitors (round through hole)
2x Radioshack 0.1uf ceramic disc capacitor (through hole)
1x Trimmable pot, not yet bough
1x Polyswitch/Reset-able Fuse ?
-IC's-
1x lp2950-33 (TO92) Datasheet
1x lp2950-50 (TO92) Datasheet
1x lt2400 (SOIC8) Datasheet
1x lm741 (DIP8) Datasheet

-Sensor-
1x Delphi Differential Low Pressure Sensor Datasheet

Eagle Schematic:

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Basic Description:

I'm using a three pin (5V,Vout,GND) differential low pressure from Delphi to measure pressure drop as an "electronic manometer".

I'm using a two regulated voltage sources, 3.3v and 5v from LP2950's.
-There is a 10k pull down resistor on output of each regulator- Background
-9v supply uses 10uf capacitor to 5.0 reg, 5.0reg feeds 3.3reg with 0.1uf capacitor, 3.3v output has 0.1 capacitor
-Delphi sensor is fed by 5.0 regulated, common ground, Vout to LM741
-LM741 is setup for inverting voltage, since the sensor outputs negative voltage for negative pressure.
-I breadboarded this setup, with the inverting opamp gain set at 1 (rf/rin->10k/10k) and found that output ranged from 3.2v @ differential 0psi - max 1.2v @ differential -0.54 psi
-LM741 fed with 5v regulated, and will be setup with a trimmable pot, since I'm not sure what gain I will need.
-LTC2400 setup like in the Background, vcc fed by 5.0 regulated, Vref is fed by 3.3, since output of the sensor ranges from 3.2v-1.2v. F0 is set to 60hz rejection, since I live the US (right?)

Here are my questions:

  1. It was suggested by dlloyd in the background thread to run a polyswitch as a current limiting device. I'm having a hard time finding a good reference for selecting a voltage / wattage. Does anyone have any recommendation or suggestion?

2)Anyone have recommendations for power supplies? I've hunted through my electronics drawers, but nothing that was below 15-18v.

  1. I replace the 5v regulated and op amp with an INA125p that I have, but it doesn't produce non-inverting output, and I can't feed the LTC2400 negative voltage to Vin. Is there a way to invert the voltage without another opamp?

4)The LTC2400 can measure mV with precision and accuracy. Over 2v (3.2-1.2) I can have 2000 steps- do I really even need an amplifier, besides for going from differential->single ended signal and/or inverting voltage?

4)In the future I would like to add additional sensors to the circuitry, and not have to add in additional ADC's. Particularly, I would like to add a small linear (rod type) potentiometer and air temperature sensor, like a thermistor/rtd/thermocouple. I can I just add in a multiplexer and reuse the same ADC? LTC2400 datasheet recommends the LTC1391.

Any criticism appreciated! I'd like to know before I spend the time building the PCB.

2

Make sure your decoupling capacitors are mounted right next to your ICs.
Is the 10uf going to ground?
Place a .1uf cap on the input of the 5 V regulator (close the the reg.) to ground.

3

Thanks LarryD!

LarryD:
Make sure your decoupling capacitors are mounted right next to your ICs.

Sounds good. Should I do this for the regulators too?

LarryD:
Is the 10uf going to ground?

All of the capacitors go to ground.

LarryD:
Place a .1uf cap on the input of the 5 V regulator (close the the reg.) to ground.

To be sure I understand, I already have a 10uf cap on the 9v Vin line after the input to 5v regulator, should I also put a 0.1uf cap before as well?

Thanks!

4

To be sure I understand, I already have a 10uf cap on the 9v Vin line after the input to 5v regulator, should I also put a 0.1uf cap before as well?

Yes, prevents oscillation if the 9v source is at a distance from the regulator.
From 9v input to ground and as close as possible to the regulator.

5

Thanks for the clarification LarryD!

Making some progress here.

In regards to question #1, this is what I've come up with.
Referencing this video, here, it looks like I need to worry about hold and peak amps, as well as temperature.

According to the datasheet for the LP2950, I didn't find a maximum amperage in, but I did find a maximum output amperage of 100mA.

The power draw on the LP2950, looks like this:
LTC2400 - 0.2mA
LM741 - 0.1mA
LP2950-33 - unknown, but probably small
Delphi Sensor - 10mA
10k pull down resistors & Caps? - unknown, but probably small

So a maximum of 100mA, and a nominal of <15mA, I'm selecting the RXEF005, with a hold current of 50mA and trip current of 100mA at ~70 degrees F.

Does this look right?

6

Answering my own question #2 here.

I converted an atx psu for use with various voltages, including 12v+, 12v-, 5v, 3.3v.

There are a ton of tutorials on this out there, and I choose a separate power supply since I see MOSFET's and motors in my future :wink:

I won't be running this through the vin on the arduino, but that shouldn't be a problem.

768×1024 58.6 KB

Next step is to order some parts on digikey.

7

Got the parts in from digikey yesterday and updated the schematic and made the board image, and tried to keep the capacitors near their relevant components.