Why do I need input OpAmps at AD7768?

Hi Fabi5 ,

The ADA4945-1 is a fully differential ADC driver that provides signal conditioning. It is suited for single supply-operation and is the recommended fully differential amplifier to drive the AD7768/AD7768-4 or the AD7768-1.

May I know what is your input type, input range. source impedance, or sensor used? Also, can you provide the target performance and specifications of your system such as accuracy or resolution? Does it required AC performance? May I know also the target throughput so that I can provide you a more concrete solution.

Regards,

JC

I use an acceleration sensor with output signal 0,5-4,5V or +/- 4V output with low output impedance. I also will add an 4-20mA input which is selectable via jumper. What do you mean by AD performance?Impedance input is maximum 1000Hz. Resolution of 16 bit is enough.

Hi,

Just to clarify, is your input fully-differential and what is the range of the absolute values of your input? Are your inputs pure DC? Lastly, what is the resistor that you will be using for the 4-20mA input? What do you mean about the impedance input is 1000Hz?

The AD7768 has a precharge buffer to ease the driving requirement of the external network. Having high input currents will lead to large errors. You can refer to the Analog Input section on page 56 of the datasheet  https://ez.analog.com/data_converters/precision_adcs/f/q-a/559381/why-do-i-need-input-opamps-at-ad7768.

Though the AD7768 has a precharge buffer, it is not a full buffer and does not replace the external driver opamp. Full settling of the analog inputs to the ADC requires the use of an external amplifier. You can also refer to this link ez.analog.com/.../if-i-enable-the-analog-input-buffers-on-the-ad7768-or-ad7768-4-ad7768-1new-ad7761-do-i-still-need-an-external-adc-driver-opamp.

Regards,

JC