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Monday, March 16, 2015

Digital Programmable Gain Amplifier

I read your post which is about, Digital gain control of Op-amp.  I have some questions to ask, if possible.  Please reply ASAP.

Precision Amplifier with Digital Control  (only for DC and low audio frequency)

Mail from KAM of UAE

Reply by delabs - Ask, i will try to reply.

I am student and I have a lab project to work on. It is about a digitally controlled variable gain amplifier. So, is your post relates to my topic. If you know a simple way of design, help me please. I will appreciate it.


See these links and tell me if this is what you are looking for. These are for Communications.

LMH6517 Digital Controlled Variable Gain Amplifier Evaluation Board

LMH6517 Digital Controlled Variable Gain Amplifier Evaluation Board

The LMH6517EVAL evaluation board is designed to aid in the characterization of TI`s High Speed LMH6517 Digital Controlled Variable Gain Amplifier

MAX2027 IF Digitally Controlled Variable-Gain Amplifier

The MAX2027 high-performance, digitally controlled variable-gain amplifier is designed for use from 50MHz to 400MHz.

The circuit i have on my site  is for low frequency and DC. used in instruments.  What you may need is for RF.  What is the Application DC, AC, or RF ?

Reply by delabs

My input is an AC signal, and I want to have changeable gains, I mean that one time the gain should  be 50,  100, 150, and 200.  However, this changeable gain should be controlled by digital inputs.  So, I think this is what the circuit do.


Gain of 200 means you may need more than one stage of amplification.The circuit of mine can be used for DC and Lower Audio Frequencies.  

If you go above 2K Hz the Slew Rate and The bandwidth of the Amp IC must be chosen carefully. These are the areas you have to study and choose. Then this circuit with fast opamps can cover Audio Frequency. 

At each stage you must study the Bandwidth response and plot a curve and use compensating caps if required. In case you want a uniform gain over a wide range over a wide range 

TDA9901 Digital programmable gain amplifier

If this is for a Very Wide bandwidth measuring instrument, it is very difficult. You may have to use special low capacitance switching elements. I can only give you ideas and links, i cannot design a circuit for you. You have to figure that out, that is how you learn. Use the internet more and study the circuits and concepts and ask me the doubts you have.

Reply by delabs

I really do not want you to design it for me. I want to learn.  The good news is that is i manage to design everything, but, I got confused when it comes to control the variable gain amplifier digitally.  I tried to use decoder and multiplexer, but, it does not work.  Can you give me some advice.  Sorry for annoying you, and thank you for your gentle help.


See this

LTC6910 - Digitally Controlled Programmable Gain Amplifiers

The LTC6910 family are low noise digitally programmable gain amplifiers (PGAs) that are easy to use and occupy very little PC board space.

Low Noise AC Amplifier with Programmable Gain and Bandwidth

Two LTC6910 programmable gain amplifiers can be combined to provide independent gain and bandwidth control using two 3 bit digital words. The Gain Control PGA simply varies the circuit gain from 1 to 100. T

Friday, March 13, 2015

Blind Temperature Controller

message: from SJ

for my hobby purpose (pewter small models) i 'm building a kind off rubber press with two hot plate about max: 200degre C. i found your website and studied the different circuits concerning J, K thermocouples, and proportional control that will suits my needs.

I would need more information about one :

- Is the circuit called Blind controller designed for J thermocouple ? as not specified, is not what component value should be adapted?.

- what is function of pot 1K RKE 1W ?

-what does mean RTD i couldn't figure out...?

- Is it possible to use at end of the controller an IC type MOC3041 after Q1 to have zero voltage switching and if so what components should be adapted or add\ removed?

my knowledge in the field are limited question could appear simple... Thanks for understanding

Mail from SJ

In this page Industrial Process Control Circuits

There is a circuit blind proportional temperature controller That is probably what you are referring to.

Blind Temperature Controller
It is for J Thermocouple you can calibrate it with boiling water and Ice. Or if you have the millivolt source you can calibrate it with that.

The 1K RKE 1W is the Dial Potentiometer, You have to fix a dial and Knob. Calibrate the Dial with markings of temperature.

RTD is not for this circuit. Unless you want to use it. A 1mA Source is shown in circuit for an 100 ohm RTD sensor.If you are using J type Thermocouple then that is not required.

RTD is more precise, but may be a bit expensive.

MOC3041 can be used, circuit is here. SSR and Zero Crossing Opto-Coplers

Remove the Relay K1. Take two wires from the points that were going to K1 Coil pins 1 and 2. Or take two wires from across diode D1, Observe proper polarity or MOC3041 may go.

The other side of MOC3041 controls Triac and is high voltage and can give shock. Buy and use a SSR

SSR and Zero Crossing Opto-Coplers
Let me know if you have any other doubts.


A words to thank you for the few minutes you spent in answering my questions, " Bravo" for your web site it is particularly well designed and functional...

Yes i might have to contact you again but your explainations are crystal clear so i should not "bother" you more at least for this project.

It is still a long path for me before my project get done, as i built myself all parts of the press,but i won't forget to E-mail you a picture of the result with details.

I could have bought one already made of theses as their price is still affordable, but my opinion is in the internet spirit and lead me to : do it yourself if you can, satisfaction will be your salary. thanks again for your help,

Reply from SJ

Thursday, March 12, 2015

Mains Voltage Monitor

i need a circuit design to detect generator voltage and display with LED for certain value of voltage. for example red LED"s for voltage under 180 V, green LED for 180-220V and Yellow LED for voltage above 240 V. i would like be glad if you would like to help me design it!

mail from KB

I have some designs that may do that easily. Have a look at them. We could modify them to suit your needs.

First the 230V has to be brought down to say 2.3V with a transformer or attenuator. Then we have to measure it with LED bar graphs, choose color of LED as required. If the measurement unit is a sealed plastic box then it can be transformer less.

Look at these first. Mains Stabilizer Circuit, In DMM Circuits see ....analog and 3-1/2 digital display for +/- 199.9mV. in LM3914 you can light up a bar graph or a single LED to show the Level.

in Water Level Sensors see Fluid or Water Level Sensor using reed relays - circuit example

DPM based on ICL7107 see AC voltmeter digital, 7107, pdf After you looked at all the circuits, Tell me how i can help you.

DPM based on ICL7107


i have seen almost all the source you show me. i got more explanation and also more confusion...!
well, actually all i need is not too complicated but maybe its tricky for me. i got the idea about using voltage comparator via voltage divider, not using transformer like you said. it is more reliable and easier, isn"t? but again i got little bit trouble to design it!

is it easy for you ? i hope you will give me best solution for it! if you dont have mind i will wait for your design as comparison, but not in hurry! Ok, thank you for your kind attention!

mail from KB

I will make a circuit for you as soon as possible. Do you need just 3 LEDs or more. do you have experience in using 230V supply safely as it can give shock. give me more details if possible.


Again thank you very much for your attention on my question. actually it will be better to have 9 LEDS for 3 kinds. i have some experiences in installing 220/230 V but not expert, dont worry about that i will try as safe as possible.

i would like to install picohydro for my garden with power about 300 watt using induction generator. it will be expensive and not comparable to use complete controller as well. so i wondering to have voltage indication to estimate voltage condition, or even you have experiences with picohydro or minimum requirement for it (controlling, safety) like overvoltage protection, etc. again i will be glad for it....!
i will wait for your answers.

mail from KB

Can you please explain in brief what is picohydro and how induction generator is used. I have attached a circuit, It is not tested, It might do the job. You must have basic knowledge in building, testing and troubleshooting circuits.

After a study of the circuit you can tell me more clearly your needs.


Ok, induction machine can also used as generator if you at least:

1. Drive motor faster than synchronous speed (slip = negatif)
2. There is reactive power from outside (capacitor/Grid)
I think you know it! in my case i used 3 phase induction motor with 1 phase output only (C2C connection),its more reliable and much better than 3 phase output for small power! its easy to build! any way, thank you for your design...!i will try to study it before i build.

mail from KB

The circuits you asked for with some explanation has been published here.

Mains Voltage monitor using LM3914

Simple Mains Voltage monitor