ADC Transfer Functions

7 Series FPGAs and Zynq-7000 SoC XADC Dual 12-Bit 1 MSPS Analog-to-Digital Converter User Guide (UG480)

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ADC Transfer Functions

The ADCs have transfer functions as shown in Figure 2-2 and Figure 2-3 . These transfer functions reflect unipolar and bipolar operating modes, respectively. All on-chip sensors use the unipolar mode of operation for the ADC. Users can optionally configure the external analog input channels to operate in unipolar or bipolar modes (see Analog Inputs, page 21 ).

For the ADCs to function as specified, the power supplies and reference options must be configured correctly. The required package ball connections are shown in Figure 1-2 . Other important aspects to ensure optimal ADC performance are the PCB layout and external component selection. These issues are covered in Chapter 6, Application Guidelines . It is recommended that you read this chapter is before the board design is started.

Figure 2-1: Status Registers

X-Ref Target - Figure 2-1


Note: The ADCs always produce a 16-bit conversion result. The 12-bit data correspond to the 12 MSBs (most significant) in the 16-bit status registers. The unreferenced LSBs can be used to minimize quantization effects or improve resolution through averaging or filtering. See Figure 2-1 .

Unipolar Mode

Figure 2-2 shows the 12-bit unipolar transfer function for the ADCs. The nominal analog input range to the ADCs is 0V to 1V in this mode. The ADC produces a zero code ( 000h ) when 0V is present on the ADC input and a full scale code of all 1s ( FFFh ) when 1V is present on the input.

The ADC output coding in unipolar mode is straight binary. The designed code transitions occur at successive integer LSB values such as one LSB, two LSBs, and three LSBs, etc. The LSB size in volts is equal to 1V/2 12 or 1V/4096 = 244 µV. The analog input channels are differential in nature and require both the positive (V P ) and negative (V N ) inputs of the differential input to be driven. More details on the analog inputs and the kinds of inputs signals that can be accommodated are covered in the Analog Inputs section.

Figure 2-2: Unipolar Transfer Function

X-Ref Target - Figure 2-2


Bipolar Mode

When the external analog input channels of the ADCs are configured as bipolar, they can accommodate true differential and bipolar analog signal types (see the Analog Inputs section). When dealing with differential signal types, it is useful to have both sign and magnitude information about the analog input signal. Figure 2-3 shows the ideal transfer function for bipolar mode operation. The output coding of the ADC in bipolar mode is two’s complement and is intended to indicate the sign of the input signal on V P relative to V N . The designed code transitions occur at successive integer LSB values, that is, one LSB, two LSBs, three LSBs, etc. The LSB size in volts is equal to 1V/2 12 or 1V/4096 = 244 µV.

Figure 2-3: Bipolar Transfer Function

X-Ref Target - Figure 2-3