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# Doubt regarding 1) -ve voltage input to AD7864 2) 12-bit Word generated by the AD7864

Hi,

I would be very grateful, if someone could answer my questions.

1) I am using the AD7864 adc(schematics have been attached). Though i am not giving any -ve voltage as Vcc (My Vcc is +5V only) i am able to get even -ve cycles of sinewaves or whatever -ve voltage i give as input to pins VINAx at my output( my Output is the output of DAC AD5447).

How is this possible??

According to my understanding, for Bi-polar operation of any device both +Vcc and -Vcc must be present. If possible please clear my doubt.

2) The datasheet says that For the ±5 V range, 1 LSB = 10 V/4096 = 2.44 mV.

What word does this (2.44mV) correspond to??

What word will be generated if +5V is given as input to the ADC??

Any help or any link to a document will be greatly appreciated ;-)

best wishes

janmay

• Hi Janmay,

Apologies for my delayed response.

I am assuming that you are using the AD7684-1. From your schematic with VinxB connected to AGND,the AD7684-1 is configure to have a +/-10V Range input voltage range.

1. On your first question that you are getting an output without giving an input to the VinAx pin. Please make sure that the analog input are at zero level by connecting the VinAx to AGND. Then check the ADC output. The corresponding output idealy should be all zeros. Please refer to page13 table 5 of the datasheet.

2. In an ADC, in every digital code has corresponding range of analog input. for example code 000 has range of analog input from 0V to 2.43mV, the next digital code would be code 0001 with voltage ranging from 2.44mV to 4.87mV. As you can see the voltage range on each digital code is equal to 2.44mV. Each digital code has a measure of 1LSB and the size of each LSB (code) for the case of AD7684 for +/-5V range is equal to 10V/4096 = 2.44mV.

From your schematic configuration with VinxB connected to AGND(+/-10V range), +5V on the analog input would have a digital code of 0100 0000 0000.

You may check the Analog Devices website there are documents available for your reference. Please also check the link below to findout more regarding ADCs.

http://www.analog.com/en/data-conversion-knowledge-resource/conversions/index.html

Regards,

Jonathan

• Hi Jonathan,

Thanks a lot for your exhaustive reply. Due to your answer to the second question, i was able to check and verify whether the ADC was producing the word for the required voltage i was giving it(by using a AD5447). And the ADC is behaving as per the datasheet words given in the table.

I would be grateful to you, if you could answer my following questions:

1)How can  the AD7684-1 take as input a -ve voltage when according to my understanding, for Bi-polar operation of any device both +Vcc and -Vcc must be present.

2) How can this device take as input(+/- 10V) when its supply range (AVdd and DVdd) can take a maximum  input of +7V (Page 6).

best wishes

janmay

• Hi Janmay,

The Bipolar ADCs can take analog input from negative voltage to positive voltage. Checking the diagram of the AD7864-1 figure 1 of the datasheet, each Vinx pin has a signal scaling circuit. The signal scaling stage is then followed by the track and hold amplifier. This signal scaling attenuates the input voltage using the resistors thus allowing the AD7864-1 to have an input from +/-10V without damaging the part.

The details of the signal scaling is discussed on page 13, Analog Input section of the AD7864 datasheet.

Regards,

Jonathan

• Hi Jonathan,

Thanks for your reply. I will look into the datasheet again and reply back if i have any more doubts.

best wishes

janmay