AMC7891_14 Datasheet, PDF (23/42 Pages) Texas Instruments – Analog Monitor and Control Circuit with 10-Bit, Multi-Channel ADC and Four DACs, Temperature Sensor, and 12 GPIOs

AMC7891
www.ti.com
SBAS518A – AUGUST 2011 – REVISED DECEMBER 2011
ADC OPERATION
The AMC7891 has two analog-to-digital converters (ADCs): a primary ADC and a secondary ADC. The primary
ADC consists of an 8-channel multiplexer, an on-chip track-and-hold, and a successive approximation register
(SAR) ADC based on a capacitive digital-to-analog converter (DAC). This ADC runs at rates up to 500 kSPS and
converts the uncommitted analog channel inputs, AIN0 to AIN7.
The analog input range for the device can be selected as 0 V to V_REF or 0 V to (2 × V_REF). The AMC7891 has an
on-chip buffered 2.5V reference that can be disabled when an external reference is preferred. The secondary
ADC is a part of the on-chip temperature sensing function.
PRIMARY ADC OPERATION
The following sections describe the operation of the primary ADC. The temperature sensor ADC always operates
in the background.
ANALOG INPUT FULL SCALE RANGE
The values in register ADC_gain determine the full-scale range of the analog inputs. The full-scale range for
input channel AINn is V_REF when bit adcn_gain = 0, or 2 × V_REF when adcn_gain = 1. Each input must not
exceed the supply value of AV_DD + 0.2 V or AGND – 0.2 V.
When internal ADC reference is enabled, the buffered internal reference is used as the ADC reference. When
external ADC reference is selected, an external reference voltage applied to the REF pin is the ADC reference.
ANALOG INPUTS
The AMC7891 has 8 uncommitted single-ended analog inputs. Figure 26 shows the equivalent input circuit of the
AMC7891. The (peak) input current through the analog inputs depends on the sample rate, input voltage, and
source impedance. The current into the AMC7891 charges the internal capacitor array during the sample period.
After this capacitance has been fully charged, there is no further input current. The source of the analog input
voltage must be able to charge the input capacitance to a 10-bit settling level within the acquisition time. When
the converter goes into hold mode, the input impedance is greater than 1 GΩ.
In applications where the signal source has high impedance, it is recommended to buffer the analog input before
applying it to the ADC. The analog input range can be programmed to be either 0 V to V_REF or 0 V to (2 × V_REF).
With a gain of 2, the input is effectively divided by two before the conversion takes place. Note that the voltage
with respect to AGND on the ADC analog input cannot exceed AV_DD.
AV _DD
AIN0
AV _DD
50 W
40 W
40 pF
AIN7
50 W
Device in Hold Mode
AGND
50 W
40 W
Figure 26. ADC Equivalent Input Circuit
40 pF
ADC TRIGGER SIGNALS
The ADC can be triggered internally by writing to the adc_trig bit in register AMC_config. When a new trigger
activates, the ADC stops any existing conversion immediately and starts a new cycle. For example, the ADC is
programmed to sample input channels 0 to channel 3 repeatedly (auto-mode). During the conversion of channel
1, a trigger is activated. The ADC stops the conversion of channel 1 immediately and starts the conversion of
channel 0 again, instead of proceeding to convert channel 2.
Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): AMC7891
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