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  1. G

    Why two types of input impedance in laboratory instruments?

    Thanks for the replies! What about the parallel capacitance then? Is it needed or it is a parasitic capacitance?
  2. G

    Why two types of input impedance in laboratory instruments?

    Hi, in some laboratory instruments (i.e. Network analyzers) the provide two kind of input impedances : 50Ohm and 1MOhm // 20pF. I understand the 50 Ohm , is the standard for maximum power transfer but why the 1MOhm parallel with 20pf? Where is this needed and how it is exactly implemented...
  3. G

    Interfacing microcontroller to 1GBPS laser driver

    All VCSEL laser drivers that I could find operate from 1GBPS. The MAX3798 is prefect for my application since it is both receiver and transmitter and it has programmable bias and modulation current. Can someone propose another driver or mixer ic? Otherwise maybe I should try and design my own...
  4. G

    Interfacing microcontroller to 1GBPS laser driver

    I don't need to use the full 1GBPS Bandwidth, just a few MBPS is fine, but the laser driver IC is for 1GPBS applications. Do u think a Serializer/Deserializer (SERDES) will help?
  5. G

    Interfacing microcontroller to 1GBPS laser driver

    Dear all, I want to interface a microcontroller with the MAX3798 laser transceiver ic. The problems are: 1. The MAX3798 has CML (current mode logic) for transmit and receive data but the microcontroller uses 3.3V or 5V voltage logic. 2. The minimum data rate of MAX3798 is 1GBPS but the...
  6. G

    Laser diode and photodiode circuit

    Actually I found a driver ic with similar topology: Any ideas how to implement it with discrete components?
  7. G

    Laser diode and photodiode circuit

    I was thinking something like this for adding the modulation current: Will the switch cause spikes in the pulses? What Voltage controlled current source do you propose?
  8. G

    Laser diode and photodiode circuit

    Hi all, I want to control the Bias and Modulation current of a laser diode using a microcontroller, a DAC and discrete components (including operational amplifiers). Current range 3mA to 25mA with 0.1mA step. Current pulses: rise time few nanoseconds, 10% max overshoot I have found this...
  9. G

    Linear power supply series transitor

    Thank you for the answer. The R7, R8 will remain constant. I will not use a pot to change the voltage but I will control the V2 from 0 to 5V with a DAC and microcontroller. Basically, because of the 1/10 voltage divider whatever i put at V2 gets multiplied by 10. ie. Vout = 10*V2 Q2 is used...
  10. G

    Linear power supply series transitor

    I am trying to figure out this simple configuration. The idea is, controlling the V2 (vref)voltage to control the Vout. I am thinking of using a microcontroller and a DAC to create the V2. How do I select the resistors values, is every resistor needed? Is the opamp + - inputs ok, or I need...
  11. G

    Linear power supply series transitor

    One more question: How can a adjustable linear supply be controlled down to 0 volts? I see many commercial supplies claiming that.
  12. G

    Linear power supply series transitor

    Thank you for your answer. What I need to know is, how do I calculate the base current depending on the Vout. i.e. when Vout=15V and I want Vout = 16V, I have an error of 1V. How does this translates to additional or less current to the base? Ib= K*(Vdes-Vout). How I calculate K? End when...
  13. G

    Linear power supply series transitor

    Hi, In linear power supplies there is a transistor (usually darlington) that is current-controlled depending on the output voltage. The problem is I cant seem to find the Vout - current-to-darlington-base equation. Do I operate it in the linear region? When Vout < Vout_desired do I saturate...
  14. G

    Crowbar circuit question

    Dear all, I don't understand how the crowbar circuit achieves short circuit in input rails without damaging the switch it uses. It seems it short-circuits the input on purpose, but then wont the device (ie. mosfet) will be destroyed?
  15. G

    Orcad Pspice digital control loop programming

    Hi, I want to implement a digital controller for a switch mode power supply in Orcad pspice. The controller will take voltages and current values as inputs and create the control signal using intelligent algorithms (state machines, digital control techniques etc). Therefore I need a block...
  16. G

    Problem with filtering noise

    Hi all, I have a sensor with a Bandwidth of 10Hz to 50kHz. I have observed that sometimes I get noise spikes at 20Hz and 60Hz. I believe it is because of the long wire of the sensor and the noisy (industrial) environment. At first I though of using a high pass or notch filter to remove the...
  17. G

    Low-cost , low-jitter clock source for 65MHz ADC

    Well, in ADS5553 datasheet is says : "its recommended to use a clock source with low jitter" , so that's why I left that part out. The only data they give on jitter is Aperture jitter of the ADC = 300fs. So I guess its not only me that I am lazy but Texas instruments too. Thanks for the help...
  18. G

    Low-cost , low-jitter clock source for 65MHz ADC

    Hi, I m looking for a low-cost, low-jitter clock for High-speed ADC. The ADC evaluation boards of different manufacturers assume that I can connect a good clock source to the input of the board so they dont provide a solution. Is there smth that I can use?
  19. G

    Variable gain input for high-speed ADC

    I think I start to understand. So there are 2 options. 1. Do smth similar to the following schematic and just have some relays to select the resistance Rf and Rg in order to control the gain/attenuation. I would prefer to control only the Rf but from the equations I see that the Gain and...
  20. G

    Variable gain input for high-speed ADC

    I mean since I will have an opamp in the circuit, I could use the same opamp for both amplification and attenuation by digitally selecting the Rf resistor. Does this make sense and how this is implemented? I want to avoid a resistor attenuator at the input since: it adds noise, extra...

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