AD7779 datasheet(48/97 Pages) AD

48 / 97 page

AD7779

Data Sheet

Rev. 0 | Page 48 of 97

SAR DRIVER

CONTROL LOGIC

FIFO

ON-CHIP

DIAGNOSTICS

SPI

AUXAIN+

AUXAIN–

AVDD4

AVSS4

CONVST_SAR

MUX

DEGLITCH

SAR ADC

REF

Figure 105. SAR ADC Configuration and Control

Table 31. -Δ Diagnostic

Input

Voltage

Recommended Voltage Reference

Notes/Result

Floating

Not applicable

Floating

Not applicable

280 mV differential signal

Internal/External

PGA gain calibration

External reference, positive/negative

External

Positive full scale

External reference, negative/positive

External

Negative full scale

External reference, negative/ negative

External

Zero scale

Internal reference, positive/negative

Internal

Positive Full scale

Internal reference, negative/positive

Internal

Negative full scale

Internal reference, positive/ positive

Internal

Zero scale

External reference, positive/ positive

External

Zero scale

Σ-Δ ADC DIAGNOSTICS (SPI CONTROL MODE)

The AD7779 -Δ ADC diagnostic functions are accessible

through the SPI interface. The internal mux placed before the

PGA has different inputs, allowing the user to select a zero-scale,

positive full-scale, or negative full-scale input to the -Δ ADC,

which can be converted to verify the correct operation of the

-Δ ADC channel.

The diagnostic mux control signals are shared across all the -Δ

channels. Depending on the diagnostic selected, connect the -

Δ ADC reference to a different reference source to guarantee

that the conversion is within the measurable range.

There are two different ways to enable the diagnostic mux:



Setting the CHx_RX bit. This bit enables the input -Δ

mux. The multiplexer inputs are described in Table 31. The

reference used during the conversions are controlled by the

REF_MUX_CTRL bits.



Setting CHx_REF_MONITOR. This bit has the same effect

as enabling the CHx_RX bit and selects the VDD1x/

AVSSx supplies as the main reference.

If the AINx± pin is connected to AVSSx, the input range is

outside range (AVSSx + 100 mV); therefore, results may differ

slightly from the expected value.

The inputs can be used alternatively to calibrate gain and offset

errors.

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