Analog Circuits - PCB surface leakage current

The latest opamps in the market have good offset performance and very low bias currents. The bias current will be of the order of the 1pA and even of the order of fA. Take for example the latest precision opamp LMP7721 from Texas Instruments which has input bias current as low as 3fA. This is very low and requires very good layout design to take advantage of the good offset and drift performance.

                Leakage effect is one of the primary concerns in such circuits. The leakage can be through the PCB crosstalk which can easily draw more current than the input current drawn by the opamp pins. This is more prevalent in the high temperature and high voltage applications. Over a period of time the dust, humidity and other impurities formed over the PCB aggravates the problem and should be taken care with good layout practices. The impurities that we are talking here can be a flux residue which is left over due to improper PCB cleaning techniques. So, the following procedures are must in such critical applications:

1.       Surface coating on the PCB to avoid humidity, dust accumulation

2.       Using high quality dielectric materials

3.       Proper PCB cleaning techniques

4.       Good layout techniques

Example scenario:

Take for example a PCB trace whose impedance w.r.t given trace nearby is 110K and a 3.3V is applied across the main trace. In this case the current passing across that junction would be 30uA. If the input bias current of an opamp is comparable with this current, then definitely the circuit will misbehave. In such cases we have implement the following technique to avoid that leakage:

Have a guard trace around the trace carrying low current. This guard trace must have a potential very close to main trace. The below snapshot shows guard ring recommendation for the LMC6001 Ultra, Ultra-Low Input Current Amplifier.

Zigbee vs Zwave

PCB Fabrication and SMT Assembly - 2

For a beginner in electronics circuit design, there is always a bit of confusion regarding the manufacturing costs of PCB. We will always be looking for a common place where we can find the costs involving PCB fabrication and also factors to be considered while designing the same. So, once the layout engineer is ready with his Gerber, the designers will start looking for various fab options. They will be sending the Gerber to fab houses to get a quotation for the PCB fabrication. During the information exchange between companies and fab houses lot of technical information gets exchanged which finally decides the PCB fabrication cost. Following are the few points you have to look at before the PCB fabrication cost is decided.

1. PCB dimensions

2. Quanity required

3. Layer count of PCB

4. Fabrication material

5. PCB thickness

6. copper foil thickness on the PCB

7. Any special treatment required for the PCB

8. Trace width and line-to-line spacing

9. Via count and dimensions

10. Special PCB sizes

CNC machine

Are you a start up in the electronics industry? Tired of turn around times for the PCBs from the manufacturing houses? Thinking of doing a PCB prototypes by yourselves with a small setup? Thinking that fabricating a PCB using chemicals can be a messy job? Then CNC machine must be one of your choices.

               CNC short form for computer numerical control. These types of machines work automatically as per the instructions provided by the user. The manual techniques used in high production environments are long one and are replaced by CNC machines. Consider the PCB fabrication process where milling is done to create the signal routing patterns. PCB drilling, routing and engraving are done using CNC machines which can bring the prototypes out quickly and at a very low cost if done in-house. The CNC machine comprises of the mini computer or the microcomputer that acts as the controller unit of the machine. The existing CNC machines involve inputting a PCB Gerber where in the complete PCB process will be automatically done through a single machine. The entire process can be monitored over a well-defined GUI. The options when using a CNC machine is used involves checking of PCB material to be etched, drill bit to be used on the machine, the PCB tools (in fact output of PCB tools) supported by the CNC machine. the drill depth can be automated in the tool. The drill position is controlled through a well calibrated motors which meant no chance of any error. The RPM of the machine can be up to 110,000. Copper clad board is used instead of photo-resistant board. Here we are eliminating hazardous chemicals during the process. The process with CNC machine might be slow than normal etching but is always a quality and better environmental friendly option.

Few CNC machine vendors are:

1. Indus, INDIA

2.  Zen Tool works

3. Accurate CNC

4. Sahajanand

5. Advanced Technocracy Inc.

There are several manufacturers from China to check out.

High speed Designs - Part 3

Transmission lines is the first term when you hear while starting to work on high speed designs. Transmission line In PCB terminology, transmission line is a trace that connects various chips on the board. The transmission circuit is generally visualized as RLC circuit. The frequency response of any circuit depends on the R-L-C elements and they become predominant when used at high frequencies. 

Traditionally, engineers used interfaces like SPI, I2C, UART which are low speed interfaces. These interfaces didn't have issues with transmission line effects of the PCB traces. Unless the signals are routed over permitted lengths, there is no issue with maintaining the integrity of the signal. As signal frequencies increased, beyond 100MHz, with interfaces like Gigabit Ethernet, DDR, PCIe, etc the transmission line effects have to be considered. We generally read in design guidelines that the signal can't be routed for example, more than 1 inch and also the signal should have a recommended routing in the PCB, etc in the case of high speed signals. So, for a high speed signal even if you route shortest but don't take care of the recommended PCB routing guideline, you might end up with signal having signal integrity issues. Some of the major issues that we come across while working with high speed designs are:

Impedance mismatch

Reflections - overshoot, undershoot, ringing

cross-talk

Radiation

The basis of signal transmission comes from Maximum power transfer theorem of electrical circuit. This theorem states that the maximum power will be transferred from source to load when the source impedance matches the load impedance. If there is a mismatch, then power transferred to load gets reduced which meant there is more loss. In high speed design, when such scenario comes up, the source tries to send the signal to load and load will not be able to absorb the complete signal. Some part of the signal is reflected back to the source. This is what causes the reflections on the PCB. the signal when reflected back travels to source is reflected as there is a mismatch again. The signal hence forth travels between source and load while degrading over time. When these occur, the reflected signal adds/negates with the original signal causing the actual signal amplitude to increase/decrease. This is what we call undershoot/overshoot in high speed domain. 

PCB Fabrication and SMT Assembly - 1

Listed are the few tips for your PCB fabrication and assembly.

CEM-1, CEM-3 as base material for PCB is low cost and not widely used now. this is not used any more. Following are the current material categories for the printed circuit board: 

• FR4, 
• High TG FR4, 
• Halogen Free Material, 
• Aluminum based material, 
• Copper based material, 
• High frequency material,
• PTFE material
• Heavy copper foil
• Paperphenolic plate
• BT
• PI
• Composite material
• PTFE + metal based
• Roger HF material

2. When finished copper thickness is equal to or more than 3 OZ, immersion gold as surface treatment is advised.

3. If the solder mask color is white, the PCBA boards may need washing.

Our PCB Tech. has good pre-sale design service and also good after-sale service. Please, do send your queries using below link:

https://ourpcb.wufoo.com/forms/instant-quote-form/

 ----- This post is contributed by Lillian from OurPCB Tech. Ltd

Battery charging Basics - 3

In the previous scenario, we have seen battery life time before it must be replaced or before it must be recharged. The condition was that there is a constant current drawn by the load for an hour. In a real time scenario, that is not the case. There will be a sudden current drawn by the load and it remains in sleep for a long period of time. This can be a case in latest IoT (Internet of things) products.

The battery calculations are done this way:

Let us assume, the following conditions for the circuit shown in the previous post:

Switcher output = 1V

Load Current = 1A

So, total power = 1W

For a given efficiency of switcher of 80%, Input power = Output Power/Efficiency = 1/0.8 = 1.25W

For a power of 1.25W at the input, current  = Power/Voltage = 1.25/7.4 = 0.169A

So, in addition, let us assume a condition when the load is drawing current of 0.169A every 10 minutes in a given hour. Considering this scenario, the current of 0.169A will be drawn 7 times an hour and negligible current for the other time.

So, in such case average current drawn by circuit from load is, 0.169*(7/(7+6)) = 0.091A

So, a 7.4V battery with 3200mAh battery capacity supplying 0.091A to load for 1 hour can withstand for, 

3.2/0.091 ~ 35 hours 

The commonly used  terminology here is the duty cycle which is calculated by the formula, T_ON/ T_ON+ T_OFF