AD7779 datasheet(47/97 Pages) AD

47 / 97 page

Data Sheet

AD7779

Rev. 0 | Page 47 of 97

256 kHz. If consecutives conversion are performed in the SAR

ADC, read back the result from the previous conversion before

a new conversion is generated. Otherwise, the results are

corrupted.

The SAR ADC is only available in SPI control mode. To read

conversion results from the SAR ADC, set the SAR_DIAG_

MODE_EN bit. After this bit is set, all data shift out from the

SDO pin are from the SAR ADC register, as shown in Figure 104.

The CONVST_SAR signal can be internally deglitched to avoid

false triggers.

Table 29. SAR Synchronization and Deglitching

CONVST_

DEGLITCH_DIS

Effect on CONVST_SAR

CONVST_SAR goes directly to the SAR

CONVST_SAR reaches the SAR when it is

1.5 MCLK cycles wide

Increase the acquisition time by 1.5/MCLK when the deglitch

circuitry is enabled.

Prior to the SAR ADC, the AD7779 contains an internal

multiplexer. This multiplexer can be configured over the SPI

interface to set the inputs to the SAR ADC to be either internal

circuit nodes in the case of diagnostics or to select the external

AUXAIN+ and AUXAIN− pins.

Along with converting external voltages, the SAR ADC can be

used to monitor the internal nodes on the AVDD, IOVDD, and

DGND pins, and can monitor the DLDO and ALDO outputs.

Some voltages are internally attenuated by 6, and the resulting

voltage is applied to the SAR ADC, as shown in Table 30. This

is useful because variations in the power supply voltage can be

monitored.

The input multiplexer of the SAR is controlled by the GLOBAL_

MUX_CONFIG register, and the different inputs available are

described in Table 30.

The SAR ADC also contains an ADC driver amplifier, as shown

in Figure 105. This amplifier settles the SAR input to 12-bit

accuracy within the t33 time. This driver amplifier helps

minimize the kickback from the SAR converter to the global

diagnostic mux input circuit nodes.

Use the auxiliary inputs, AUXAIN+ and AUXAIN−, to validate

the -Δ measurements. While operating in SPI control mode,

the AD7779 has three available GPIO ports controlled via the

SPI interface. The GPIO pins can be used to control an external,

dual 8:1 multiplexer, which in turn is used to sample the eight

-Δ channels. Use this diagnostic in applications where functional

safety is required. This diagnostic aids in removing the need for

a secondary external ADC to validate primary measurements

on the -Δ channels.

Temperature Sensor

The internal die temperature can be measured with an error of

±2°C. DVBE is proportional to the temperature measured

referred to 25°C.

Temperature (°C) =



Table 30. SAR Mux Inputs

SAR

Input

Positive

Signal

Negative

Signal

Attenuation ÷ 6

AUXAIN+

AUXAIN−

DVBE

AVSSx

REF1+

REF1−

REF2+

REF2−

REF_OUT

AVSSx

VCM

AVSSx

AREG1CAP

AVSSx

Yes

AREG2CAP

AVSSx

Yes

DREGCAP

DGND

Yes

AVDD1A

AVSSx

Yes

AVDD1B

AVSSx

Yes

AVDD2A

AVSSx

Yes

AVDD2B

AVSSx

Yes

IOVDD

DGND

Yes

AVDD4

AVSSx

DGND

AVSSx

Yes

DGND

AVSSx

Yes

DGND

AVSSx

Yes

AVDD4

AVSSx

Yes

REF1+

AVSSx

REF2+

AVSSx

AVDD4

Yes

SDI

SDO

SET BIT 5

GENERAL_USER_CONFIG_2 REG

WRITE TO ADC MUX REGISTER

ADC CONVERSION RESULT REG

Figure 104. Configuring the AD7779 to Operate the SPI to Read from the SAR ADC

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