Design Inquiry

Question

Hi Team, 
 I'm planning to use the ADC IC AD7768 for my Analog Card. For that i need a differential input amplifier. For me the input to the amplifier is single ended signal coming from the mixer IC AD630ARZ. 
 Can the amplifier ADA4940-1 works on single ended inputs to differential pair outputs, so that i can convert my single ended signal to diff pair that to be fed into the ADC IC. 
 For ADA4945-1 I can provide V+ 5V and V- -5V or as recommended in datasheet +7 and -2V. 
 I'm planning to provide AVDD1 - 5V, AVDD2 - 5V, IOVDD - 1.8V as power supplies for the ADC. For this requirement, i want input range +/-4V for ADC. 
 My required input voltage range to the ADC is +/-4V. 
 The block diagram is shared below. 
 
 For the above mentioned supply configuration for ADC and input amplifier does it can provide +/-4V input voltage range for ADC IC and whether ADC can accept those input range values with this supply configuration. 
 Please provide an update on above query. 
 
 Regards, 
 Abhishek

JEstayo · Answer

Hi AbhishekManoj , 
 The ADC can accept +-4V based on the supplies you have sent. Please refer to the FAQ below on the analog input range of AD7768. 
 AD7768: What is the analog input range? - Documents - Precision ADCs - EngineerZone 
 The amplifiers ADA4940 and ADAD4945 can work using single ended to differential output to drive the ADC. Please refer to the link below for suggested ADC drivers using different ADCs. 
 Precision ADC Driver Tool | Analog Devices 
 Thanks, Janine

JEstayo · Answer

Hi AbhishekManoj , 
 The output of the ADA4940 or ADA4945 is differential. The differential limit in this case is +-4.096. Please open the excel file to verify the limits for every pin - AIN+ and AIN-, and the limit differentially (AIN+ - AIN-) 
 
 Please refer to the answers below.

These are internal to the IC. The AIN pins are the analog input pins of the ADC. 
 The symbol shows that the ground pin of the SMB is connected to the EVB ground. 
 The components should be provided. Please refer to the Precision ADC Driver tool to confirm the correct RC filter for the kickback. 
 You can connect to VCM_OUT. Please try to explore the excel file for the input range of the ADC using different supplies, and input configurations. 
 The VCM of the ADC is by default set to half of Vref. Please connect this to the VCM pin of the amplifier before the ADC to set the common-mode to VREF/2. 
 The VCM_OUT of the ADC is buffered using OP2177 and connected to the amplifiers to level shift the signal and set the common mode output to VREF/2. 
 AVSS can be connected normally to ground. 
 In pin mode, pull the pins up to IOVDD, or pull down to ground. 
 You can connect to analog ground. 
 For CMOS, connect CLK_SEL to ground, apply the clock to pin 32 and tie pin 31 to DGND. Please refer to the CLOCK SELECTION in page 58 of the datasheet for more details. 
 You can use this to ground XTAL1 pin. 
 
 Thanks, Janine

JEstayo · Answer

Hi AbhishekManoj , 
 For point no. 8, 0 or 10kohm can be used. When 0 ohm, be careful not to connect the pin to both IOVDD and DGND, 
 For point no 10, You can connect the clock signal to XTAL2 directly. 
 For unused analog input pins, you can short those to VCM. For other pins, you can refer to PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS section on where to connect if unused. 
 Thanks, Janine

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