CODEC AD1836A  ADC verification

CODEC AD1836A AD converter working in differential input mode: ( Gain=1)

We have:

ADC2 Left Input 1=2.25V +small AC signal

ADC2 Left input 2=2.25V

Differential mode setting and gain=1.

The Data sheet says

Full-Scale Input Voltage (Differential) 2.18V ( RMS), 24 bit ADC

My question is :

(1)  If differential input voltage= -0.1V, what is the ADC2 digital output? Is ADC2 unipolar or bipolar?

(2) If differential input voltage=0.1V, what is the ADC2 digital output?

(3) if the differential input voltage is constant DC voltage, then will the ADC 2 output  be zero?

We have experience with other ADC, such as AD7714, but not familiar with ADC for CODEC.

we would like understand, verify and test how CODEC ADC works.

Thanks

  • 0
    •  Analog Employees 
    on Jan 9, 2018 2:13 AM

    Hello blackfin,

    These are audio specific ADCs where the 7714 is designed for data conversion so there are other options. The 1836A's ADCs are sitting up at a common mode input DC level. The ADC will produce a 2's compliment 24 bit output. So if the input of the ADC is sitting at the 2.5V common mode, it will produce a zero output. (now this is ideal, in reality there is always a DC offset since no converter is perfect.). So 2.5V will produce zero. A slight positive DC signal above 2.5V will produce a positive number. The max is 0x7FFFFF. Note the sign bit is low.

    For a slightly negative number, just one bit negative will produce 0xFFFFFF which is a minus 0x000001 in 2's compliment form. So the maximum negative number possible is 0x800000. Note the sign but is high.

    So if the input is a constant DC offset then it will output a constant number. It can be a positive or negative number depending on the DC voltage as it compares to the common mode voltage of ~2.5V.

    Dave T

  • Second question for AD1836A:

    The datasheet says

    " Full-Scale Input Voltage (Differential) 2.18 (rms) -----------6.16V (V pp)"

     "Vref=2.25V ( common mode)"

    in our differential mode design,  assuming Gain=1, when the differential input voltage is   3.08V,  ADC output will be saturated to the most positive-MAX number , and when the differential input voltage is    -3.08V,  ADC output will be saturated to most negative- MIN number?

    Thanks

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:00 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin