Re: Driving ADC, please help

Hi Ramon,

You could use a basic inverting opamp config with the ADA4841-1

http://www.analog.com/static/imported-files/tutorials/MT-213.pdf

Rin would be twice Rf. You could use 5V and ground as the supplies. However the limitations are you may lose some range near ground and near 5V (maybe 50-100mV depending on head/footroom spec). Also the input impedance is dictated by Rin, so not a high impedance input. Is the positive supply fixed at 12V?

Best Rgds,

Alan

  • You could also leave the power supply on the op-amp at +12, configure the gain to give a 0-10volt output for your given input range and use a 2:1 resistive divider on the output. This also gives a measure of protection to the input stage of the ADC. In order for the resistors to be configured at a low enough value you may need to use a power op amp or a power boost stage. I successfully used this approach in a 20bit audio ADC I designed in the 90's ('Optimising the Performance of 20-bit audio ADC's', preprint 4262 , 100th AES Convention, Copenhagen, 11-14 May 1996)

  • The requirement is just a single supply (not dual supply), 12V is just a random voltage.

    I choosed 12V because I thought that supply voltage must be equal or greater than input signal (10V).  Can we directly use 10V as input signal and power the opamp with just 5V?

    (Sorry, I divided the thread and cannot join it again)

  • David, Thanks. I don't have access to your paper but I have seen something similar at the input stage of AD7671.

    It uses four input pins (R, 2R, 4R, and 4R). For 0V-10V (and 2.5 Vref) both 4R's inputs are connected to Vin and both '2R & R' connected to ground.

    So voltage divider is 2R || (2R||R).

    According to datasheet R = 375 ohms.  (4R = 1500 ohms, 2R = 750 ohms)

    So input voltage of 10V is divided by four (750 ohms || 250 ohms) to give 2.5V (as Vref).

    The problem is that I am not sure how to configure the opamp. I need a single-supply opamp stable at 0.5 gain. Which configuration is better for stability? inverting or non-inverting? I don't need high speed (around 1Mhz input signal).

  • Hi Ramon,

    The solution i described is a single supply solution. You use a 5V supply for Vs+ and connect the Vs- to ground. Because of the inverting config you have a resistor from the 10V to virtual ground on the opamp so the opamp itself does not see 10V.

    Best Rgds,

    Alan

    Edit: Sorry Ramon, an important piece of info I left out is that you will need to bias the + input of the opamp at 2/3 of the output range or 3.33V in order to level shift the opamp output. This could be achieved with a 2:1 resistor divider from the 5V reference. In terms of stability see this RAQ

    http://www.analog.com/static/imported-files/rarely_asked_questions/RAQ_attenuator.pdf

    Message was edited by: Alan Walshe

  • Hi Alan.

    ADA4841-1 at 5V supply has only an input voltage range of -0.1V to 4V. So, I am not sure what would be the available signal range of 10V input signal after level shifting to 3.33V.

    I am looking now at AD8275 (level translator) and AD8475 (funnel amp). AD8275 has 0.2x gain (1/5), and AD8475 has two gain (attenuation) options of 0.8x and 0.4x. But datasheet only talks about bipolar inputs and differential outputs. I want just only to convert a unipolar input and get single ended output.

    Maybe I could just use a 1/4 precision resistor divider network and buffer? What do you think?