Post Go back to editing

Scaling issues on EVAL-AD4111/12SDZ

Thread Summary

The user is experiencing incorrect readings on the EVAL-AD4112SDZ when measuring 0-5V unipolar signals via SPI. The issue was resolved by enabling the input buffers and using the internal 2.5V reference, but the 5V signal reads at 20% of full scale due to a fixed voltage divider. The AD7173-8 is suggested as an alternative ADC with a wider input range to maximize the 5V signal.
AI Generated Content
Category: Hardware
Product Number: AD4112

I am using the EVAL-AD4112SDZ to read three unipolar signals (0-5V) . I am communicating with the eval board via SPI with a microprocessor, rather than using an SDB board. I have verified my SPI comms are working correctly. I have set the registers to be the following:

ADCMODE 0x2000

IFMODE 0x0000

SETUP0 0x0030

FILTER0 0x0510

CH0 0x8010

CH1 0x8030

CH2 0x8050

When the channels are connected to VCOM I see ~ 0x042000, when channels are connected to VDD (5V) I see ~ 0x0B4000. This is the same as when I use the Eval board's 2.5V VREF (with LK1). I would expect to see 0x000000 and 0xFFFFFF respectively. I have tried to recalibrate and the values look the same. Is my configuration incorrect? Is there some voltage dividers on the eval board I am not accounting for?

Thank you

  • Hi  ,

    First, I noticed that you are not enabling the input buffers. The Analog input buffers should be enabled for voltage Analog inputs as these provide a low source impedance to the sampling ADC. If these buffers were disabled, then a large resistance would exist. This combined with any parasitic capacitance from the resistive front end driving the ADC input will result in a very large gain error.

    Additionally, when a 5V input is connected, this will not result to 0xFFFFFF due to the voltage divider on the analog front end which has a division ratio of 10 enabling an input range of ±10 * Vref.

    Hope this helps.

    Regards,

    JC

  • Ok thank you. Enabling the input buffers and changing my voltage reference to internal 2.5V helped. Now my 5V signal is reading at 20% of full scale which makes sense. Is there any way to configure the voltage divider to have 5V reach full scale? Besides decreasing the Vref? Or is there another ADC you would recommend? 

  • Hi  ,

    With the AD4111/12, there is no way to set the 5V as the full scale and the voltage divider is fixed to have an attenuation factor of 0.1. You may need to use an ADC with lower input range. Will you be needing the both the current and voltage inputs? If you only require voltage inputs, then you may want to check the AD7173-8 (Rev. B). It has an input range of +/-Vref only so you could maximize the range of the ADC.

    Regards,

    JC

Before You Switch


Switching languages will make ADI Explorer unavailable. Resume your session by switching back to English and reopening ADI Explorer.