ADS7047: how to use Diff. ADC as Pseudo single ended ADC

Hi Deepa,

The ADS7047 is a unipolar ADC so I am not sure what you mean by the -ve pin. Can you clarify? You are correct in that it is fully differential. Using the ADS7043 may be an option here as this is a Pseudo-Differential device in the same family. 

Multiple ADCs may also be used here as shown in the TIDA-01355 design. 

Regards,

Aaron Estrada

Hello Aaron,

we have singleended sensor output. ADS7046 was selected but there is no distributor available for this product. hence we are switching to 

this ADS7047 is written as FD and Fully Differential Input Range: ±AVDD. so problem is that how to connect this FD to SE sensor.

thanks,

deepa

Hello Aaron,

it is good to hear that ADS7047 is a fully differential ADC. how it will behave if -Inn is connected to GND. due to budget &space issue we can not afford another chip.

Maxim, & Analog sites are suggesting to connect the "Diff sar adc" to signal ground which allow DC common-mode voltages to be canceled. i hope that similar is possible with TI adc aswell.

br,

Deepa 

Hi Deepa,

You cannot tie the AINM pin to GND. The ADS7047 has a common mode ((AINP - AINM)/2) requirement of AVDD/2. If AINM is tied to GND, there will be cases where the common mode requirement will be violated causing bad data out. In order to tie the minus input to the ADC, you will need a "True Differential" ADC. A True Differential ADC allows a wider common mode range, typically 0V to Vref. 

Regards,

Aaron Estrada

Hello Aaron,

to avoid the THSxxx, can I connect the 1.65v to AINM ? where Vref/AVDD =3.3v.

what will be the restriction if AINM is constant? can i get the FScale=0 to 3.3v?

thanks,

deepa

Hi Deepa,

You can do this with a pseudo-differential ADC such as the ADS7043. If using the ADS7043 you can also get the full scale range from 0V to 3.3V. 

Regards,

Aaron Estrada

can i use three DE ADC in parallel to get the higher sampling rate....?? similar to  TIDA-01355

Hi Deepa,

Yes, this is possible. However, there are some considerations mentioned in the reference design that should be noted. I have also included a link below to an article about interleaving SAR ADC's that I think will be helpful. 

https://www.ti.com/lit/pdf/slyt731

Regards,

Aaron Estrada

Hello Aaron,

thanks...

This AN is covering SE ADS7056, while I will using the DE ADS 7047. how the extternal RC filter need to calculate? do I need to connect the +ve pin together to sesor high side while -ve pins to GND of sensor?

BR,

Deepa

Hi Deepa,

With the ADS7047, you will need a single-ended to differential conversion. Why not go with the Pseudo Differential ADS7043? With the ADS7043, you can connect the sensor output to the AINP pin and tie AINM to the common mode voltage (Vref/2). 

Regards,

Aaron Estrada

Hello Aaron,

we need high sampling rate and hence we selected the ADS7046 (which can be connected in parallel to increase the sampling rate), but this device is not available. so we switch to ADS7047. now question is how can we connect more than one DE ADC in parallel to get the higher sampling rate.

would you please share the AN for DE ADC with time interleaving method.

BR,

Deepa

Hi Deepa,

The method for interleaving ADCs is the same regardless of the input architecture. So interleaving single-ended ADCs is the same for Differential or Pseudo-Differential. 

Regards,

Aaron Estrada

Hello Aaron,

for SPI bus, how much this adc can load?

is this true that CSDO = 20 pF, SPI bus will load the bus with 20pf??

(we are planning to put two adc on single bus....)

BR,

Deepa

Hi Deepa,

The typical SDO capacitance is 2.3pF. I believe Csdo is referring to the load capacitance during the Idvdd current measurement. 

Regards,

Aaron Estrada