MCU

DAC8581

Lookup

Table

(FLASH)

DVM

Board

Tester

Computer

Board

Tester (ATE)

MCU

DAC8581

Lookup

Table

(FLASH)

DAC8581

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SLAS481C – AUGUST 2005 – REVISED OCTOBER 2012

APPLICATION INFORMATION

IMPROVING DAC8581 LINEARITY USING EXTERNAL CALIBRATION

At output frequencies up to 50 kHz, DAC8581 linearity error and total harmonic distortion are dominated by

resistor mismatches in the string. These resistor mismatches are fairly insensitive to temperature and aging

effects and also to reference voltage changes. Therefore, it is possible to use a piece-wise linear (PWL)

voltages, etc. The cancellation of linearity errors also improves the total harmonic distortion (THD) performance.

It is possible to improve the integral linearity errors from ±25 LSB to ±1 LSB and the THD from –70 dB to almost

–98 dB (see Figure 17 and Figure 18). The improvements are at the expense of ~2x DNL deterioration, which is

not critical for the generation of large-signal waveforms.

Figure 19. A Simple Printed-Circuit Board Scheme for Calibrated Use of DAC8581

Figure 20. Production Test Setup for a DAC8581 Board With Calibration

The PWL calibration scheme uses a DAC8581 and a microcontroller unit (MCU) with flash memory, on a printed-

circuit board as seen in Figure 19. Calibration is done during board test, and the calibration coefficients are

stored permanently in flash memory as seen in Figure 20. An automated board tester is assumed to have a

precision digital voltmeter (DVM) and a tester computer. The test flow for a 1024-segment, piece-wise linear

calibration is as follows:

1. Use the tester computer to load software into the MCU to ramp the DAC8581 and:

–

Take a reading at each step after a short wait time

–

Store 65,536 readings in the tester computer volatile memory

2. Use the tester computer to:

–

Search the 65,536-point capture data and find the actual DAC8581 codes which would generate ideal

DAC outputs for DAC input codes 0, 64, 128, 192, … .

–

Store these actual codes in the onboard microcontroller’s flash memory in a 1025-point array called

COEFF[].

3. Use the tester computer to program the MCU such that, when the end-user provides new 16-bit input data

D0 to the MCU:

–

The 10 MSBs of D0 directly index the array COEFF[].

–

The content of indexed memory of COEFF[] and the content of the next higher memory location are

placed in variables I1 and I2.

–

The six LSBs of the user data D0 with two variables I1 and I2 are used for computing Equation 1 (see

Figure 21).

–

Instead of D0, I0 is loaded to DAC8581

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