Re: Driving ADC, please help

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?

Hi Ramon,

Please see the following tutorial pages 6,7 and 8

http://www.analog.com/static/imported-files/seminars_webcasts/High%20Speed%20System%20Applications%20(PDF)/HS%20Systems%20Part%202%20for%20Print_A.pdf

On page 7 you want to use config C as i have described. In this scenario there is no issue haivng 10V inputs with a 5V supply. Both opamp inputs will sit at 3.33V. This config has been used in this circuit note

http://www.analog.com/static/imported-files/circuit_notes/CN0254.pdf

You could use a resistor divider followed by a buffer stage if you wish. Make sure the buffer supply has enough headroom for the signal range you require.

Best Rgds,

Alan