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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1353 of 1441
NXP Semiconductors
UM10503
Chapter 48: 12-bit ADC (ADCHS)
48.7 Functional description
48.7.1 Analog inputs
The A/D convertor architecture is a flash convertor type with differential inputs. The
nominal output code is:
d_out = 2048 x (vin_pos - vin_neg)/400 mV
The output format can be set by TWOS to either two's complement or offset binary.
The input signals of the ADC can be either AC-coupled by means of two capacitors or
connected directly to the inputs (DC-coupled). For AC coupling, the DC biasing is
generated internally by setting DCINNEG and DCINPOS to 1.
The input channels are multiplexed to ADC input vin_pos. All six input channels share the
same vin_neg, which is generated internally when DCINNEG is set to 1 and output at pin
ADCHS_NEG. The nominal vin_neg level is 0.50 V. Hence vin_pos should be in the range
from 0.50 ± 0.4 V.
It is possible to deselect the internal vin_neg and supply vin_neg externally at pin
ADCHS_NEG by setting DCINNEG to 0. When only one input is used then the ADC can
be used in true differential mode.
When sampling different channels in a high-speed interleaved fashion some crosstalk
may occur when one input is immediately sampled after another input. This crosstalk
effect increases with an increased ADC clock frequency and an increased source
impedance driving the input. It can be reduced by various means:
•
Driving the affected input with a lower source impedance.
•
Decreasing the ADC clock frequency.
•
Increasing the ADC clock frequency by at least 2x and increasing the time between
two successive samples to more than 1 descriptor timer count (inserting one or more
dummy cycles).
48.7.2 Reduced power modes
The ADC power consumption is proportional the fADC. Besides reducing the fADC
frequency to reduce power the ADC has a power down mode and a power-down mode to
reduce power.
In power down mode (PD_CTRL = 1) power consumption is reduced, but not zero.
Recovery to normal mode is fast; RECOVERY_TIME/fADC.
In power-down mode (POWER_SWITCH=BGAP_SWITCH = 1) the ADC power
consumption is zero but recovery to normal mode takes more time;
RECOVERY_TIME/fADC plus additional 100 µs for BGAP and 10 µs for POWER.
Table 1150.Interrupt 1 set status register (SET_STAT1, address 0x400F 0f34) bit description
Bit
Symbol
Description
Reset value
29:0
SSTAT1
Interrupt set status
0x00000000
31:30
-
Reserved.
-