Could ADC inputs support the negative voltages? - Page 1
Forum section: electronics / projects

author: javad2040
author_rank: Contributor
timestamp: June 05, 2018, 03:16:28 pm
content: Hello guys.I decide to design a data acquisition system with High-speed ADC such as AD9643 or AD9684 and Xilinx FPGA.My question is : could this ADC support the negative voltage values in inputs and how to design the system that support both negative and positive values?I search about this subject and fine one solution that is using the one DAC and PGA in ADC inputs that adds the input values and voltage references that comes form ADC and conect to ADC InputsThanks.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 05, 2018, 03:20:23 pm
content: You have to bias the input such that with a zero volt signal, the input port sits at halfway the supply voltage.

author: Zero999
author_rank: Super Contributor
timestamp: June 05, 2018, 06:18:11 pm
content: Quote from: javad2040 on June 05, 2018, 03:16:28 pmYes, the best way to do this depends on the signal.Is it AC, i.e an audio signal? Then all that's needed is some biasing resistors and a coupling capacitor.Is the voltage swing of the signal greater, than that of the ADC input? If so then it'll need to be attenuated, which can be achieve with a potential divider and biasing resistors.Is the voltage range of the signal just right or does it need to be increased? An op-amp will be required to shift and amplify the signal to the optimum range for the ADC.

author: bson
author_rank: Unknown
timestamp: June 05, 2018, 06:30:07 pm
content: The input needs to be within the two reference voltages; so either you need to supply a negative reference, or you need to bias the input.However, ask yourself if you really need to sample negative voltages, often this isn't necessary at all and a differential ADC might work just as well. In many cases when you have a sensor (such as, for example a current sense resistor in some configurations) a differential input will do the job without a need for a negative reference or biasing.

author: tszaboo
author_rank: Super Contributor
timestamp: June 05, 2018, 06:50:19 pm
content: The AD9643 does support negative voltages. If IN+ is larger than IN- your input is positive, otherwise your input is negative. But remember, that voltage is always relative to something. In this case both inputs are positive, relative to the GND in the system. TI has a quite comprehensice AD input buffer collection, select the one you need.http://www.ti.com/lit/an/sloa098/sloa098.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2018, 06:57:36 pm
content: Some ADCs apparently can read negative values, the ATxmega for instance (I've read it in the datasheet, but not used it yet).Differential ADCs, like pictured, read voltage around an elevated common mode, so that you don't need any negative voltages in circuit. They also usually provide signed or unsigned formatting. RTFDS.Tim

author: javad2040
author_rank: Contributor
timestamp: June 13, 2018, 09:18:16 am
content: thank you for reply.One goal of this project is capture the exponential signals that is less than zero(negative value), but this design should be flexible to support the both positive and negative values, using a DAC to produce relevant offset has recommended and i decide to use this method.My new problem is that the AD9684 ADC Datasheet and Evaluation board schematic only offers the AC-couple inputs for ADC and no referred to design the DC-couple inputs.please help me to solve the design of DC-couple inputs for AD9684.Regards,

author: capt bullshot
author_rank: Super Contributor
timestamp: June 13, 2018, 09:48:10 am
content: Quote from: javad2040 on June 13, 2018, 09:18:16 amObviously, that's for a reason.This chip is targeted at the RF communication segment, and there's no need for DC coupling. Most of it's specifications are AC performance related, that's another dead giveaway. So this chip would be fine for some kind of SDR application.That doesn't mean that you cannot use DC coupling, but you're on your own. It's input is self-biasing at 2.05V. So you'll have to provide a DC coupled differential signal that sits on a 2.05V common mode level. Most fully differential OpAmps can do that for you, just connect the common mode set pin to 2.05V.