# Single-ended to differential conversion and ADC application

I am not quite familiar with the things I am going to describe. So, I need some help over here.

I want to integrate an AD4001 ADC at a board I currently design. The thing is that my analog signal is single-ended and I need it to be differential to enter the ADC. The datasheet of the AD4001 proposes an application circuit, the one at the figure I attach.

Based on the AD4001 datasheet, I selected the ADA4940 differential amplifier for converting my analog signal to its differential one. I downloaded the SPICE model from its site and ran it using LTSpice. Attached is its circuit.

My analog input signal is 0-5 Volts, so I used a voltage supply with 2.5V DC offset and 2.5V magnitude. Then, 2.5V as VOCM and 5V as VS. The ouput signals I get are the following:

Is the blue-red differential output the right one of the green input? How is it connected to the VOCM? I cannot understand how the initial signal can be formed based on its differential one.

Is it the correct input for the ADC for my analog input? I have read that the ADC converts the difference between positive and negative signals, but their difference never reaches 5V.

Also, I would like a single supply system (5 V DC). But my analog signal is 0-5V. Does this mean that it is not feasible to implement such a circuit since Vref reaches ~4 V or I could connect 5V to Vref and V+ of the amplifier?

• Why are you using a differential A/D when the signal you are trying to measure is already ground-referenced? This makes no sense. – Olin Lathrop Jul 27 '17 at 13:18
• I want an SPI interface integrated in the ADC and a throughput as close to 5 MSPS as possible. I found the AD4001 with SPI and 2 MSPS and thought it matches... – Konstantinos Tsoumanis Jul 27 '17 at 13:21
• That's a silly reason to pick a otherwise inconvenient part. There should be many SPI A/Ds out there to chose from. Also, can you simply ground the negative differential input of the A/D? I haven't looked at the datasheet to see if this would be possible. Often differential input is a option, not a requirement. – Olin Lathrop Jul 27 '17 at 13:45