Analog Input ADS2807 EVM

Aimen,

If I understand correctly, you want to know why your board has R5, R6, R7, and R8 installed when the schematic says they should not be? If they are installed, check to see if R9 and R11 are installed. Ont his board, it appears that R9 and R11 were used to terminate the source, but my guess is that these were removed and replaced with R5, R6, R7, R8, which provide the same termination, but closer to the part.  If R9 and R11 are installed, then remove R5, R6, R7 and R8.

The full scale input voltage range for the part is 2Vpp or 3Vpp depending on if the SEL pin is low or high, respectively.

-Matt Guibord

Thank you Matt G.

//The full scale input voltage range for the part is 2Vpp or 3Vpp depending on if the SEL pin is low or high, respectively.

Wehn I have the 2Vpp configuration, this signifies  the Signal at J5 and J6 between 0V und 2V.

Aimen,

The ADC inputs are biased around the common mode voltage provided by the CMA and CMB pins.  If you look at the schematic of the EVM, you'll notice that the CMA and CMB pins are connected to the center tap of the corresponding input transformers, providing the required common mode voltage to the input pins. The common mode level is Vs/2 (2.5V for a 5V supply). Using a differential input, as is set up on the board already, each input pin will swing 1Vpp around the common mode voltage (which equals 2Vpp differential).

-Matt Guibord

Hi Matt

I remove R5 R6 R7 and R8 and I give in J5 Sinus Signal between -200 mV und 200 mV. but he d'ont work. When I connect the Signal to J5 the Signal will 0V??

Aimen,

What input frequency are you using?

-Matt Guibord

The Frequenz Input is 666 666 Hz

Aimen,

As in 666.666 kHz? Can you clarify what you are seeing at the input to the ADC? What point are you measuring at? Is the rest of the board in the default configuration?

-Matt Guibord

Matt,

I have only Replaced the Output logic buffers SN74ABT16244A with SN74ALB16244DGGR.

I have done the following measurements

between J5 and C11 : -40 mV

between C11 and T1 Pin 6 : 0V

at Pin 3 from T1 : 2.5V

at Pin 2 from T1 :2,5V

at Pin 1 from T1 : 2,5V

Aimen,

Are you using a DMM or oscilloscope to measure these points? J5 and C11 should be the same potential, same with C11 and T1 Pin 6. The common mode voltage is correct on pins 1, 2, 3 of the transformer. You need to measure from C22 or C23 to ground.

-Matt Guibord

Matt,

I use oscilloscope

I mean with  //between J5 and C11 : -40 mV// . I mesuring between J5 and C11 to GND.

The Voltage from C22 to GND is 2,5V.

The Voltage from C23 to GND is 2,5V.

Hi,

For sinus signal I use LM358 , there has 20 mA Output.

Is the 20 mA Current  enough for the Transformer?

Aimen,

Since you have no signal at J5, this leads me to believe that your error lies somewhere else, such as with the LM358.  Verify that you have a sine wave coming out of the amplifier before attaching it to the ADS2807.  The LM358 has a gain-bandwidth product of 1MHz.  Unless you're using this amplifier as a unity gain buffer, the amplifier is not fast enough for your 666.666kHz signal and is likely the problem.

-Matt Guibord

Matt,

I have mesuring sine Wave[-200mV to 200mV] befor attaching it to the ADS2807, when I attaching it the Signal will dc -20 mV.

Aimen,

I'm going to have to see your schematic and screenshots of measurements in order to help you.

-Matt Guibord

Matt,

I have replaced the LM385 with AD8028. I change the circuit, the Signal is quadrate.

I mesured between 100 Ohm und J5 from ADC to GND:

befor I connect the cable to ADC the Voltage range is [0V  2V]

after I connect the cable the Voltage Range is [ 0V   0.5V]

Hi,

In Datasheet ( ADS280x-EVM) the name from the  Transformer in image is MCL102 T1-1T but im Board is MCL422 T1-1T installed.

Is it the same?

I mesuring the resistance from Transformer, he is 2 Ohm .

Im Datascheet from KK81 T1-1T the resistance is 50 Ohm.

??

Aimen,

You are seeing a significant voltage loss at the input to the ADC because the ADC EVM has a 50 ohm termination on the board. This forms a voltage divider with the 100 ohm resistor you have at the output of your amplifier.

The easiest way to fix this would be to remove the 100 ohm resistor and drive the ADC input directly. Since your input frequency is relatively low, this solution should work fine.  You might need a DC blocking capacitor on the output of the amplifier if you have any DC offset.

Regards,
Matt Guibord

Hi,

I am testing with function generator.

The Signal from function generator befor I connect it to ADC :

Sinus

Vpp=1V

frequency = 4 MHz

resistance = 50 Ohm

I connect the Output from function generator and I mesuring at R9 to GND.

Vpp=50mV

Why the tension will smaller after to connect it to ADC?