AD7771 datasheet(52/99 Pages) AD

52 / 99 page

Data Sheet

AD7771

Rev. 0 | Page 51 of 98

The SAR ADC has a maximum throughput rate of 256 kSPS.

The CONVST_SAR pin initiates a conversion on the SAR ADC.

The maximum allowable frequency of the CONVST_SAR pin is

256 kHz. If consecutive conversions 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 shifted out from the

SDO pin originates from the SAR ADC conversion, as shown in

Figure 121.

The CONVST_SAR signal can be internally deglitched to avoid

false triggers.

Table 29. SAR Synchronization and Deglitching

CONVST_DEGLITCH_DIS

(Register 0x013, Bits[7:6])

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 AD7771 contains an internal

multiplexer. This multiplexer can be configured over the SPI 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 monitors

the internal nodes on the AVDD, IOVDD, and DGND pins, and

the DLDO and analog LDO (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 SAR driver amplifier, as shown

in Figure 122. 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 Σ-Δ measurements. While operating in SPI control mode,

the AD7771 has three available GPIOx ports controlled via the

SPI. The GPIOx pins can be used to control an external, dual

8:1 multiplexer, which, in turn, samples the eight Σ-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 Σ-Δ channels.

Temperature Sensor

The internal die temperature can be measured with an accuracy

of ±2°C. The differential voltage base emitter (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 121. Configuring the AD7771 to Operate the SPI to Read from the SAR ADC

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