CS5505-AS datasheet(17/40 Pages) CIRRUS

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Understanding Converter Calibration

Calibration can be performed at any time. A

calibration sequence will minimize offset errors

and set the gain slope scale factor. The delta-

sigma modulator in the converter is a differential

modulator.

To calibrate out offset error, the

converter internally connects the modulator dif-

ferential inputs to an internal VREF- voltage and

measures the 1’s density output from the modu-

lator. It stores the digital code representation for

this 1’s density in SRAM and remembers this

code as being the zero scale point for the A/D

conversion.

The converter then connects the

negative modulator differential input to the

VREF- input and the positive modulator differ-

ential input to the VREF+ voltage. The 1’s

density output from the modulator is then re-

c o rde d. The c onverter use s th e digita l

representation of this 1’s density along with the

digital code for the zero scale point and calcu-

lates a gain scale factor. The gain scale factor is

stored in SRAM and used for calculating the

proper output codes during conversions.

The states of A0, A1 and BP/UP are ignored

during calibration but should remain stable

throughout the calibration period to minimize

noise.

When conversions are performed in unipolar

mode or in bipolar mode, the converter uses the

same calibration factors to compute the digital

output code. The only difference is that in bipo-

lar mode the on-chip microcontroller offsets the

computed output word by a code value of

8000H (16-bit) or 80000H (20-bit) and multi-

plies the LSB size by two. This means that the

bipolar measurement range is not calibrated from

full scale positive to full scale negative. Instead

it is calibrated from the bipolar zero scale point

to full scale positive. The slope factor is then

extended below bipolar zero to accommodate the

negative input signals. The converter can be

used to convert both unipolar and bipolar signals

by changing the BP/UP pin. Recalibration is not

required when switching between unipolar and

bipolar modes.

Converter Performance

The CS5505/6/7/8 A/D converters have excellent

linearity performance. Calibration minimizes the

errors in offset and gain. The CS5505/7 devices

have no missing code performance to 16-bits.

The CS5506/8 devices have no missing code

performance to 20-bits. Figure 7 illustrates the

DNL of the 16-bit CS5505. The converters

achieve Common Mode Rejection (CMR) at dc

of 105 dB typical, and CMR at 50 and 60 Hz of

120 dB typical.

The CS5505/6/7/8 can experience some drift as

temperature changes. The CS5505/6/7/8 use

chopper-stabilized techniques to minimize drift.

Measurement errors due to offset or gain drift

can be eliminated at any time by recalibrating

the converter.

65,535

Codes

32,768

-1

+1/2

-1/2

Figure 7. CS5505 Differential Nonlinearity plot.

CS5505/6/7/8

DS59F4

CS5505/6/7/8

DS59F5

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