Using A Bluetooth Module vs A Chip Based Design

Bluetooth OEM modules have usually gone through all the testing so you don’t have to.  You can slap one on your PCB and your ready to go, often with a faster development route too.  The catch is that it costs more in production than using the discrete components instead.  It can be even more frustrating that the Bluetooth IC the module uses is capable of running the embedded software your application needs, but you have to use a separate microcontroller because either the Bluetooth module implementation doesn’t allow it or if you do you would break the Bluetooth stack certification, requiring re-testing.

As a general rule of thumb, if your production volume is likely to be say 10k or 20k then going with a module is often an overall cheaper choice once your factor in the testing costs.  However if your production volume is likely to be say 100k then the testing costs will typically be overcome by the savings in production cost.

The Cause

Until relatively recent times digital PCB design (and especially when prototyping) could be viewed as simply a means to electrically interconnect components and unless you designed RF circuits there was little else to worry about. However the PCB itself, or the means of connecting the components used (i.e. prototyping), is now is a very common cause of a loss of signal integrity. The reason is mainly due to the rise and fall times of output signals having decreased as devices are designed to operate faster and faster and to use smaller and smaller silicon manufacturing processes. This problem is not actually due to the operating frequency of a device or the frequency at which a signal is changing, it is due to the speed at which a signal output changes state from high to low and low to high. A signal doesn’t instantaneously change from high to low or low to high, it takes a certain amount of time which will be specified as the rise and fall time in a devices data sheet. Previous signal rise and fall times of many 10’s of nano seconds have now become times measured in just a few nano seconds or for many devices they are measured in pico seconds.

So you may be thinking, this can’t possibly be an issue for me, my board is only operating at a few MHz and I’ve even slowed my data bus down to a few KHz. Well unfortunately that doesn’t matter. If you work with a DC signal the only thing you really care about in a wire or PCB track is its resistance, which for short lengths will be close to zero. However, when using that wire or PCB track with a fast AC signal it starts to behave like a capacitor and inductor. Capacitors and inductors exhibit resistance to alternating current called reactance. The impedance of the wire or track is the vector sum of resistance and reactance, essentially the total resistance seen at a particular frequency. What happens when you send a signal with a fast rising and falling edge down a wire or PCB track, if the impedance of the gate driving the wire or track isn’t exactly the same as the one receiving the it, is it that some of the pulse bounces (literally) back to the driving gate. As there is still an impedance mismatch, the signal continues to bounce between the two until it finally dampens out. This bouncing becomes worse as the speed of signal rise and fall times increases. Basically, the faster rise and fall times of signals from modern semiconductors combined with wire or PCB trace inductance and capacitance causes noise signals of a greater magnitude than before. Greater magnitude means the bouncing signals can reach the threshold voltage required for the receiving device to ‘see’ another clock pulse, or an incorrect data level at the moment it is sampling the data line.  The solution is to design your PCB to use impedance controlled tracks on these clocked connections.

Single Connections

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Conductor Impedance

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width until you get the impedance (Zo) you need.  This is the thickness to make your tracks

100ohm Single Track Impedance (General signals, SPI bus etc)

General signals, SPI bus, etc will generally perform well with a 100ohm track Impedance.  Some examples track widths with a GND plane under the tracks:

2 layer 1.6mm PCB (1.48mm FR4) = 0.61mm (100.3150ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.27mm (100.2273ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.12mm (100.9915ohms), or if too small then 0.15mm = (95.0631ohms)

4 layer PCB Train – Internal to Internal (0.99mm FR4) = 0.4mm (99.8636ohms)

4 layer PCB Train – Internal to External (0.22mm FR4, layer 2-3) = 0.05mm – not possible

4 layer PCB Train – Internal to External (1.245mm FR4, layer 1-3) = 0.49mm – not possible

75ohm Single Track Impedance

Also relevant to 75ohm radio antenna connections.Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 1.29mm (75.0457ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.6mm (74.7938ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.3mm (74.8156ohms)

50ohm Single Track Impedance

Also relevant to 50ohm radio antenna connections.  Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 2.65mm (50.1165ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 1.25mm (50.0581ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.65mm (49.9609ohms)

Differential Pairs

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Differential Pairs

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter clearance between tracks in conductor distance
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width and spacing until you get the impedance (Zdifferentail) you need.  This is the thickness to make your tracks

USB 90ohm Differential Pair Track Impedance

USB 2.0 requires 90ohms differential impedance (max 45ohms per track)

Max trace-length mismatch between High-speed USB signal pairs should be no greater than 3.81mm.

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.38mm track width = 90.525ohms Zdifferential
35um copper, track spacing 0.2mm and 0.43mm track width = 90.174ohms Zdifferential

To stick with the max 45ohms per track (not practical for many designs):
35um copper, track spacing 1.4mm and 0.75mm track width = 89.118ohms Zdifferential

4 layer PCB Pool – Internal to Internal (0.71mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.61mm track width = 90.385ohms Zdifferential

4 layer PCB Train – Internal to Internal (0.99mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.8mm track width = 90.156ohms Zdifferential

2 layer 1.6mm PCB (1.48mm FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 1.12mm track width = 90.184ohms Zdifferential

10/100Mbps Ethernet 100ohm Differential Pair Track Impedance

Ethernet requires 100ohms differential impedance (max 50ohms per track)

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.3mm track width = 100.462ohms Zdifferential
35um copper, track spacing 0.2mm and 0.34mm track width = 100.923ohms Zdifferential

To stick with the max 50ohms per track (not practical for many designs):
35um copper, track spacing 2.5mm and 0.65mm track width = 99.835ohms Zdifferential
If you use 1 of the plane layers with GND above and below (0.71mm & 0.38mm from track to GND planes) you get:
35um copper, track spacing 1.5mm and 0.42mm track width = 99.0.16ohms Zdifferential

Ensure Your PCB IS Made With The Right Stackup

Its a good idea to include the required stackup on one of your copper layers to ensure it is used.  Something like this:

http://www.eecs.umich.edu/~prabal/projects/hijack/

Good for temperature ranges of -200 to +600ºC.

Temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. They are usually made of platinum and often called platinum resistance thermometers (PRTs).  They are often more suitable than thermocouples in industrial applications below 600 °C due to their higher accuracy and repeatability.

PT100 & PT1000

The most common type of resistance temperature sensor used in industry

100 ohms and 1000 ohms at 0°C respecitvely, increasing with increasing temperature.

Useful resources:

http://www.iqinstruments.com/iqshop/technical/pt100.html

Voltage Follower

Non Inverting Amplifier

Output goes positive if the + input is more positive than the – input.

Output goes negative if the + input is more negative than the – input.

Output will do whatever is necesasry to make the inputs equal.

Design Considerations

Opamps are very stable with temperature.

JFET (i.e. TL084) inputs can be bad for oscillations / noise as they are high impedance.  Usually need to use with resistor and capacitor on input.

Voltage Follower

A voltage follower has the output connected to ‘-’ and the input voltage connected to ‘+’.

Unused Inputs

Wire as a follower (output to ‘-’) but with ‘+’ input tied to a mid votlage (0V for dual supply, between 2 resistors for single supply).  If you have a single supply and don’t want to fit resistors then you can connect the ‘+’ input to 0V, but the op amp will consume more current and analog purists will berate you.  Alternatively if you have a voltage rail sitting somewhere between the op amps +V & – V then you can use that.

Bad Approaches

Using a resistor to 0V is not really a better solution.

Tying both inputs together can be bad due to randomness in offset votlages in an op amp

Tying inputs high and low can over stress some inputs and causes more current consumption.

Good resources

http://www.maxim-ic.com/appnotes.cfm/an_pk/1957

OpAmps We Often Use

LM324 (4 OpAmps, Single Rail)

General purpose, swings GND to (VCC – 1.5V).
Quite high output drive

LMV324 (1/2/4 OpAmps, Single Rail)

General purpose, rail to rail, 2.7V – 6V, low running current, low cost

LM358 (2 OpAmps, Single Rail)

General purpose, low power, swings GND to (VCC – 1.5V), use with logic systems

LMV721 (1/2 OpAmps, Single Rail)

Low noise, 2.2 – 5V, rail to rail, low power, 10MHz

AD8544 (1/2/4 OpAmps, Single Rail)

General purpose, rail to rail, 2.7V – 5.5V, low running current,
1MHz, instrumentation, sensors, audio

AD8531 (1/2/4 OpAmps, Single Rail)

General purpose, 250mA output, rail to rail, 2.7V – 5.5V,
Low running current, 1MHz,

TL064 (1/2/4 OpAmps, Dual Rail)

Lower power version of TL084

TL084 (1/2/4 OpAmps, Dual Rail)

JFET input OpAmp

NE5532

High power output

3V Devices to 5V CMOS

Pic input pins have a worst case voltage range of 0.8V – 4V @ 5V supply (0.2 VDD – 0.8VDD). The TTL pins have a better range than this.

4 connections:

74HCT125 is a good cheap choice (14 pin package, quad 3-state buffer).
Power at 5V. Connect each of the 4 output enable pins to 0V. The min high input voltage is 2V so 3V3 compatible and the output state matches input state (it’s a straight buffer).

74HCT08 (quad AND gate).

Many other 5V CMOS chips with TTL level input buffers may be used

8 Connections

74HCT245 or 74ACT245 also provides the solution. The HCT & ACT provide an input voltage range of 0.8 – 2V @ 5V supply. As its 5V powered its output voltage range is good for the PIC. Note that an HC or AC will not work as the input voltage range is not the same.

5V CMOS to 3V Devices

A 74LVT245 powered @ 3V3 has 5V tolerant inputs so will do the job. It will only work in this direction though. A LVT may not be powered @ 5V.

A CD74HC4050 can be powered at 2V to 6V and will accept input voltages up to +16V.

74LVC16373 – 16 bit latch, 3V3 device which accepts 5V inputs

5V CMOS to 3V Devices Bi Directional

A 74LVX4245 will do the job and is available from Farnell. Port A is 5V, port B is 3.3V. T/R and OE can be driven from either side.

A 74ACT245 combined with a 74LVT245 should also do the job and is very slightly cheaper, but obviously takes more space.

Both Ways & Single Wire (1V8, 3V, 5V)

SN74LVC1G07DBV is a open drain output mini IC package which can be powered at 1V8 or 3V (1.65 – 5.5V) and will accept +5.5V on its input or output. It can also be powered at 5V and specs a high level input signal as min 0.7 x VCC for 5V supply.  SN74LVC1G07DBV is the alternative inverting version.

If you are reducing voltage (i.e. 5V in to 3V out) you can use the SN74LVC1G126DBV without the need for a pull up resistor, which may be an advantage in low power designs.  The SN74LVC1G240DBV is the alternative inverting version.

1V8 to 3V/5V Bi Directional

74LVC8T245

Power VCCA at 1.65V to 5.5V
Power VCCB at 1.65V to 5.5V
DIR and OE are supplied by VCCA.

I2C Bus

PCA9517A provides I2C bus interfacing if you don’t want to use mosfets to do it:

Power VCC_A with 0.9V to 5.5V
Power VCC_B with 2.7 V to 5.5V
Tie EN to VCC_B to enable

Higher Voltage Interfacing

A 74HC4049 or 74HC4050 can be powered at 2V to 6V and will accept input voltages up to +16V.

4049 is basically the same but consumes a bit more current and has a min supply voltage of 3.5V.

A SN7417 is a 5V powered logic buffer which has open drain outputs that can drive loads up to 15V @ 30mA. SN7407 will do up to 30V @ 40mA and is cheaper

Logic To Darlington

ULN2803A