Post Go back to editing

High-precision digitally controlled amplifier

Hi everyone,

I am trying to build a high-precision digitally controlled amplifier in order to amplify signals coming out from AD9833. I would like to precisely amplify that output signal from around 1Vpp to 12Vpp in steps of 10mV or 20mV. AD9833 uses a 25MHz MCLK, thus the highest frequency expected is to be around 12.5MHz. However, square and triangular waves need much higher bandwidth so as not to destroy their high frequency components.

I have already been searching a voltage controlled amplifier in Variable Gain Amplifiers (VGA) | Specialty Amplifiers | Analog Devices comprehensively, but I cannot find anything that suits: I would need a wide bandwidth, linear-in-voltage, high-precision VGA (no PGA with a few fixed values). All those VGAs seem to be video signal or line-driving oriented and do not offer me a simple interface to control. I just would need a DC control pin or a serial SPI interface.

I have also thought of using an instrumentation amplifier Instrumentation Amplifiers | Analog Devices together with a digital potentiometer Digital Potentiometers | D/A Converters | Analog Devices placed as Rg to control the gain. Digital pots have low bandwidth so I cannot use them in the amplification network nor even as an attenuator, but since in this sort of amplifiers they only balance the inner current to modulate the gain perhaps will work. What do you think?

Could someone give me some advice about what to choose or the best form to achieve this challenging amplification?

Thank you for your time

Parents
  • Hi Sparkles!,

    In the configuration using in amp and digipot, the digipot will introduce low pass filter due its resistor and switches parasitic capacitance. It will low pass the signal before it goes through the in amp output. The larger the end to end resistance of the digipot the smaller the bandwidth it can handle.My suggestion is to use a DAC as the feedback resistor for an op amp configure below (as a reference) to meet your requirement. The gain of the circuit is set by adjusting the digital inputs of the DAC. You can refer to this tutorial: MT-072 for more details. The circuit of this type requires multiplying DAC with voltage output.Multiplying DAC is a DAC with wide reference voltage range. You just need to look for a perfect op amp and DAC combination to have a wider bandwidth. I think AD8027/AD8028 will do for the amplifier. It features high bandwidth and rail to rail input and output voltage range. The DAC reference input voltage swing should be wide enough to accept the op amp output voltage swing.

    Hope it helps.

    Phil

Reply
  • Hi Sparkles!,

    In the configuration using in amp and digipot, the digipot will introduce low pass filter due its resistor and switches parasitic capacitance. It will low pass the signal before it goes through the in amp output. The larger the end to end resistance of the digipot the smaller the bandwidth it can handle.My suggestion is to use a DAC as the feedback resistor for an op amp configure below (as a reference) to meet your requirement. The gain of the circuit is set by adjusting the digital inputs of the DAC. You can refer to this tutorial: MT-072 for more details. The circuit of this type requires multiplying DAC with voltage output.Multiplying DAC is a DAC with wide reference voltage range. You just need to look for a perfect op amp and DAC combination to have a wider bandwidth. I think AD8027/AD8028 will do for the amplifier. It features high bandwidth and rail to rail input and output voltage range. The DAC reference input voltage swing should be wide enough to accept the op amp output voltage swing.

    Hope it helps.

    Phil

Children
No Data