MP8798AS datasheet(7/20 Pages) EXAR

7 / 20 page

MP8798

Rev. 3.00

Accuracy of Conversion: DNL and INL

The transfer function for an ideal A/D converter is shown in

Figure 4.

Figure 4. Ideal A/D Transfer Function

3FF

3FE

3FD

VREF(–)

VREF(+)

DIGITAL

CODES

000

002

LSB

OFW = 0

0.5

∗ LSB

0.5

∗ LSB

001

OFW = 1

1 LSB

V001

V002

V3FE

V3FF

V0FW

The overflow transition (VOFW) takes place at:

VIN = VOFW = VREF(+) – 0.5 ∗ LSB

The first and the last transitions for the data bits take place at:

VIN = V001 = VREF(–) + 0.5 ∗ LSB

VIN = V3FF = VREF(+) – 1.5 ∗ LSB

LSB = VREF / 1024 = (V3FF – V001) / 1022

Note that the overflow transition is a flag and has no impact on

the data bits.

In a “real” converter the code-to-code transitions don’t fall

exactly every VREF/1024 volts.

A positive DNL (Differential Non-Linearity) error means that

the real width of a particular code is larger than 1 LSB. This error

is measured in fractions of LSBs.

A Max DNL specification guarantees that ALL code widths

(DNL errors) are within the stated value. A specification of Max

DNL = + 0.5 LSB means that all code widths are within 0.5 and

1.5 LSB. If VREF = 4.608 V then 1 LSB = 4.5 mV and every code

width is within 2.25 and 6.75 mV.

N + 1

N – 1

Output

Codes

Analog

Input

(N+1) Code Width = V(N+1) – V(N)

LSB = [ VREF(+) – VREF(–) ] / 1024

DNL(N) = [ V(N+1) – V(N) ] – LSB

Figure 5. DNL Measurement

On Production Tester

LSB

DNL

V(N+1)

V(N)

The formulas for Differential Non-linearity (DNL), Integral

Non-Linearity (INL) and zero and full scale errors (EZS, EFS) are:

DNL (001) = V002 – V001 – LSB

:::

DNL (3FE) = V3FF – V3FE – LSB

EFS (full scale error) = V3FF – [VREF(+) –1.5 ∗ LSB]

EZS (zero scale error) = V001 – [VREF(–) + 0.5 ∗ LSB]

Figure 6. Real A/D Transfer Curve

DIGITAL

CODES

0.5

∗ LSB

000

001

002

3FE

3FF

1.5

∗ LSB

V001

V002

V3FE

V3FF

VREF(–)

VREF(+)

EZS

EFS

Figure 6. shows the zero scale and full scale error terms.

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