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db09284e_V3.01_preliminary_2017-07-06

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AS89020

V3.01

Figure 6: Influence of settling time to conversion result

This example shows: The ADC result after first conversion is lower than result after second conversion. This

is why the settling time is not reached at start of first conversion.

5.7 Temperature Compensation

The implemented transimpedance resistors of the channel’s input amplifiers have a temperature coefficient

(TCR) with a typical value of -4300 ppm/K. However, the AS89020 integrates a temperature compensation

circuit, so that the temperature dependence of the transimpedance resistance is eliminated. Furthermore

the overall ADC gain is internally controlled, so that the absolute accuracy can be achieved. If the internal

temperature compensation is disabled (CREG: TCOMPENA = ,0b'), the overall behavior of the device is no

longer temperature independent. Normally, the AS89020 should be used with enabled temperature

compensation (CREG: TCOMPENA = ,1b').

However, the temperature dependence of the reference current source at REXT (generating I(REXT)) is not

compensated and therefore the temperature coefficient of the external resistor REXT of 200 kΩ directly

affects the ADC accuracy.

5.8 SPI – Interface & Communication

The digital interface of the AS89020 consists of a bidirectional four wire SPI for programming the registers

and for transferring the conversion data to the microcontroller.

The SPI interface is used for the connection of the AS89020 to the system’s SPI bus. The SPI clock SCLK

is low active and stays high while the AS89020 is in idle state. The data is changed at the time of the falling

clock edge and will be accepted at the time of the rising clock edge.

TRACKING

TRACKING

HOLD /

CONVERSION

HOLD / CONVERSION

tTRACK

settling time tSET

Input signal

change

Start of

Conversion

End of

Conversion

Start of

Conversion

End of

Conversion

Start of

Tracking

input current

output voltage of

input amplifier

output voltage of

track and hold stage

ADC conversion

result