ads804e

Hi.

The input ANA1 swings from 0V to 3.3V.

I changed R7 to 7.87K and R8 to 12K.

REFT and REFB are disconnected from VREF33 and GND ,

REFT,REFB, CM   are wired as in Figure 5 .

/IN1 is connected to GND .

Please Advise.

Ephraim Oved.

Hi,

Changing those resistor values to 7.87Kohm and 12Kohm will indeed change the full scale input range to about 3.3V, but the common mode level of the input signal must still be around 2.5V.  So if you set full scale to be about 3.3V then the input signal would have to be from 2.5V-1.65V to 2.5V+1.65V or from 0.85V to 4.15V.  Not from 0V to 3.3V.

And the /IN pin must be connected to 2.5V if the input signal is single ended. 

If the signal on ANA1 is known to be always a higher frequency then the signal could be AC-coupled to the data converter and could be biased to put the center-point of the signal at about 2.5V.  Many amplifier devices have an input pin for common mode and if you put 2.5V on that pin then the amplifier itself would output a signal centered aroud 2.5V.  Alternatively, a resistor biasing network could be used to bias the signal up to be centered at 2.5V after the AC coupling. 

If the signal cannot be AC coupled, maybe because the frequency content of the signal includes DC itself, and the amp cannot center the signal at 2.5V, then you could make the fullscale range of the data converter be 5V, and then the data converter will expect a signal from 0V to 5V and you would then just not use the full range.

But the /IN pin must still be set to 2.5V in any case. 

Regards,

RIchard P.

Hi.

I am sorry , I don't follow you.

I am confused with the /IN and common mode level mechanism.

What are my options of using this IC.

Assuming the following.

1. Using single ended input.

2. The input ANA1 is 0 to 3.3V.

How should I wire the IC ?.

Please Advise.

Hi,

i can't tell you more about how to wire your circuit unless I know more about your input signal and what you wish to accomplish.

In the datasheet, please take a look again at figures 7, 8 and 9.  These show three cases of single ended input for three different settings of full scale range.  In each case the /IN pin is connected to 2.5V.  And in each case the single ended input range is centered around 2.5V.  In Figure 7, the full scale range is 5 V in which case 0V is the minimum input and would result in an output sample code of 0000 0000 0000, and 5V is the max input and would result in an output code of 1111 1111 1111.   In Figure 8, the device is set for 2V full scale in which case the input range is from 1.5V to 3.5V.  In Figure 9, the full scale is 4V and the input range is from 0.5V to 4.5V.  In all cases, the input voltage range is centered around 2.5V.  This is called the common mode level, which is also the level the signal would have to be biased to if the input were differential instead of single ended.

There is no setting in which case the input voltage of 0V will result in sample code 0000 0000 0000 and an input voltage of 3.3V would result in a sample code of 1111 1111 1111.  An input range of 0V to 3.3V is not centered around 2.5V. 

I gave you several options for how to deal with this, depending on your input signal.   If you do not need to sample an input signal with frequency content all the way down to DC then you can AC couple the signal as in Figure 1.   Then after the AC coupling you could bias the signal to be centered at 2.5V like figure 1 does with two 2Kohm resistors to REFT and REFB.

If you cannot AC couple, then you can use the amplifier to shift your signal from 0V - 3.3V up to be in the range of 0.85V - 4.15V.   (As in Figure2.)

Or you can just make the full scale range be 5V so that your input of 0V would become sample code 0000 0000 0000 and your input voltage of 3.3V would become sample code 1010 1000 1111 and you would only be using 2/3 of the range of the device.

Regards,

Richard P.

Hi.

I can't change the input amplitude, It swings from 0 to 3.3V.

I want to use the full scale of the input .

I need to get 000000000000 for 0V and 111111111111 for 3.3V.

What modification do I need to do with the AD804 or the OP.

Please Advise.