AD7768 : 8 CHANNEL ADC CAN SUPPORT SINGLE END - BIPOLAR INPUT.

Hi Niall,

Thanks for your inputs,

As we are using 8 Channel Inputs, Each channel inputs I should convert single end to differential end bipolar signal using LTC6363. 

With your design reference discrete components occupy more spaces. 

My board dimension will be 4.5 cm circular dia.

I have another query,

1 ) If I will attenuate the signal range from 10Vpp to 5Vpp (That is AIN input signal range  +2.5 to -2.5V) signal range.

How i should connect the power inputs (AVDDx , AVSSx) and reference inputs (REF+x and REF-x)?

2) If I give differential Input signal (-2V at AIN1+ ; +2V to AIN1- ) what will be the output data? can you give me the conversion formula ? what REF+x and REF-x I should give ?

Thanks and Regards,

Mohamed Sidiq.H

Hi Mohamed,

You will need an ADC driver to achieve the same level of performance using the AD7768 as in the datasheet. So a 4.5cm board will be a challenge as you will need the ADC, reference, reference buffer and 8 x ADC drivers.

1. If I will attenuate the signal range from 10Vpp to 5Vpp (That is AIN input signal range  +2.5 to -2.5V) signal range.

   How i should connect the power inputs (AVDDx , AVSSx) and reference inputs (REF+x and REF-x)? AVDDx will need to be at +2.5V referenced to DGND and AVSSx will be set to -2.5V referenced to DGND. Thus there will be 5V between AVDDx and AVSSx. All analog circuitry will then need to be referenced to AVSSx such as the driver amplifier rails and the reference. The 5V reference will be +2.5V above DGND on the REF+x pin. REF-x will be set to -2.5V when referenced to DGND (you can connect directly to AVSS. 

2. If I give differential Input signal (-2V at AIN1+ ; +2V to AIN1- ) what will be the output data? can you give me the conversion formula ? what REF+x and REF-x I should give ? The ADC will see (AIN1+ minus AIN1-) = -4V. The output from the AD7768 is in two's compliment format. So -4V corresponds to 1110 0110 0110 0110 0110 0110 (0xE66666) with a 5V reference. A simple way to decode this is to convert to straight binary and use code 8,388,608 as mid-scale, code 0 as 0V and code 16,777,215 as +10V. You can then work in straight binary so the -5V to +5V input range becomes 0 to 10V. The picture below is taken from the AD7768 datasheet and explains the Twos Complement coding.

Regards

Niall

Hi Mohamed,

Please see my answers to your questions below:

1. Is this LDO Current is enough for AD7768 Supply? ADP7118 is used on the AD7768 customer evaluation board and so it is a proven combination. The ADP7183 is needed to create the analog ground plane at -2.5V and should not need to supply large amounts of current to the ADC. 
2. The LTC6363 can perform both single-ended to differential conversion and attenuation using the circuit I included above. Therefore this one part can be used to replace the two parts above and also achieve better performance. I would recommend using this instead of AD826 and ADA4922, especially since you mentioned board space was of concern. 

Another factor you should also consider is simplification of the 8th order low pass filter. A 4th-order filter will suffice if the ADC is operating in the optimum configuration. The neat thing about the AD7768 versus a traditional SAR ADC is that the digital filter will take care of the anti-aliasing for you. The potential aliasing points which you need to protect against are shown in the image below and depend on the speed of the modulator. If you run the modulator at a faster rate the unprotected regions shown below can be pushed much higher than the 30kHz bandwidth you require and the complexity of the antialiasing filter can be reduced. 

Regards

Niall

Hi NiallM,

Thank you very much for your inputs !!!

I am going with AVDDx, REF+x = +2.5V  using  ADP7118 ; and AVSSx , REF-x = -2.5V using  ADP7183ACPZN2.5-R7

Is this LDO Current is enough for AD7768 Supply ?

I have an rearranged architecture of my 8 Channel Receiver card.

Before the architecture is :

Re-arranged architecture :

Can you give me your clarification of following parts.

1.        AD826: I am going to attenuate my signal ranges from 10Vpp to 5Vpp (+/-5V to +/-2.5V ). By selecting RF /Rin with inverted output (Example:  RF = 1K ; Rin 2K = -0.5 Gain). This Output is fed to the input of another channel of  AD826. Hence I will get +/-2.5V attenuated signal.

        Is this right way of attenuate my signal range ? or do you have any recommended part for this design?

2.         ADA4922-1 This attenuated signal is fed to the ADA4922-1 to convert the single ended bipolar signal to   differential bipolar signal and fed to the ADC (AD7768) input.

This single ended to differential end conversion is there right part to do the conversion instead of you recommended LTC6363.

Please confirm may I go ahead with this part ?

Thanks and Regards,

Mohamed Sidiq.H

Mohamed,

I just stumbled across this post, and wanted make some comments on the signal chain you have posted on August 21.

You show the ADA4922-1 driving the AD7768 - The ADA4922 will cause performance degradation of the AD7768 at the maximum sample rate.  As the AD7768 samples the output of the ADA4922, it injects a small amount of charge onto that node at the beginning of each acquisition.  The amplifier that drives that node needs to be able to recover from that disruption quickly, and drive that node back to the proper voltage.

Niall is recommending the LTC6363 or the ADA4940 b/c they are capable of driving the AD7768 at it's full sample rate.

The ADA4922 *might* be able to drive the AD7768 when using a much lower fmod than the maximum value.  Here's a snippet regarding fmod from the datasheet:

One of the specs that's critical in this application is GBWP:

• LTC6363: GBWP = 500MHz
• ADA4940: GBWP = 130MHz
• ADA4922: GBWP = 40MHz

Faster is not necessarily better, but for this ADC, 40MHz GBWP is probably too low for the driver to fully settle between samples.

On the other hand, if you're willing to use a lower fmod, the ADA4922 might work fine.

-Anne

Hi Anne,

Greeting for the day !!!

I made a mistake in the block-diagram , In that my input is differential 10 Vpp signal which is fed to the PGA AD8253 .

The PGA converts the differential to single end convertion with selectable gain of 20db, 40db, 60db.

After this PGA I am going to remove high frequency components and allowable range of 0 -30khz Band and remove other range of signals. Hence the Output of PGA will be 20Vpp.

Due to the Input Constraints of AD7768 (AD7768 – allow only Differential or pseudo- differential Input with maximum allowable Common mode range is 5Vpp).

hence I want to attenuate this signal 20-Vpp to 10-Vpp and Then ,I want to convert this single ended bipolar signal into differential bipolar as 5Vpp to match the AD7768.

Can you suggest any other part is available to attenuate and convert single end to differential conversion chip parts ?

Mr. Niall Suggest to use LTC6363 , In this Common mode input range is 8.8V as Max if 10V supply used. and also maximum power supply is 11V.

In ADA4940 , Power supply max is 7V and Common mode input will be −VS − 0.2 to +VS − 1.2.

Due to this constrain I couldn't able to move further,

In AD4922 have 26V as max Supply and Input common mode range ±10.7 , so that I asked your suggestion may I use this part .

Please let me know if you have any suggested part for attenuate and single end to differential end conversion.

Kindle let me know if you need any clarification regarding my inputs.

Thanks and Regards,

Mohamed Sidiq.H

Mohamed,

Yes - Looking at differential drivers, it looks like there's a limited selection for higher supply voltages.  

If you are willing to slow down the sample rate of the AD7768, then the ADA4922 will settle fine.  You may run into performance issues at the fastest sample rate.

Hi ,

I am looking for 24 bit ADC. Then I saw this post.

I can see many things happing in signal conditioning before ADC. I am curious and want to know the Signal conditioning and ADC that is coming for this Post.

1. ADC7768- Since in the post, we are Differential input in the range of  ± 1.25/2.5V.

a. It means we are attenuating the signal. So is this the right way to do condition monitoring?

b. Can we use this ADC for  ±5V range?

c. Can we use AD7176-2 in place of AD7768 for ±5V range (http://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0310.html)?

In Signal conditioning-

2. We are Passing Differential signal to AD8253 (PGA) that is converting differential signal into Signal ended. Then We are using LPF- LT1697IS (Dual/Quad Low Noise, Rail-to-Rail, Precision Op Amps-) But it an Op-amp. 

So how can we use this as an LPF? Can we not use differential LPF?

then We are attenuating the signal using Attenuator (LT1678)- But As per Mohhamad suggested that why not we can go for LT6363 (It can take differential Input (±5V range), work as an LPF and generate Differential Output). So if we use this IC (LT6363) we don't need to add LPF, attenuator, and Single-ended to a differential ended amplifier like- LT1944/ADA4922-1.

So As we are making the DAQ system we should use minimum component for better performance. Kindly suggest and if I missing any point for the DAQ system. Do we have any other Differential ADC to full fill above criteria?