COP8-REF-AM datasheet(52/83 Pages) NSC

52 / 83 page

15.0 A/D Converter (Continued)

TABLE 26. ADRSTH

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Bit 9

Bit 8

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

TABLE 27. ADRSTL

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Bit 1

Bit 0

00000

15.2 PROGRAMMABLE GAIN AMPLIFIER

A programmable gain amplifier is located between the ana-

log multiplexor and the input to the A/D. It supports single

ended mode only. The gain of this amplifier is selected by the

ADGAIN register shown in Table 28. This register is also

used to enable the stand-alone amplifier (AMP1) on port pins

B3, B4 and B5, the internal temperature sensor, and the

offset trimming configuration. Both the stand-alone amplifier

and the programmable gain amplifier will draw DC current

when enabled. To minimize the amount of current drawn in

the HALT mode, the user should disable both amplifiers

before entering HALT. This register is initialized to 00h on

Reset.

TABLE 28. ADGAIN

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

EN-

AMP1

ENTS

TRIM

Reserved

GAIN2 GAIN1 GAIN0

ENAMP1 Enables stand-alone amplifier (AMP1) to port B.

Enabled = 1. Disabled =0.

ENTS

Enable internal temperature sensor. Enabled =

1. Disabled = 0.

TRIM

Configures the programmable gain amplifier into

the trimming configuration by shorting its + and −

inputs together. Enabled = 1. Disabled = 0. This

bit should be set to 0 for normal use of the

programmable gain amplifier.

GAIN2:0

Controls the gain of the programmable gain am-

plifier. See Table 29. When performing a conver-

sion on the temperature sensor, a gain of 1 or 2

must be selected, depending on the operating

voltage of the device. A gain of 2 can only be

used for the temperature sensor if V

≥ 4.5V.

Reserved These bits are reserved and must be 0.

TABLE 29. Gain Bit Assignments

TRIM

GAIN2

GAIN1

GAIN0

Gain Tolerance

0000

Not Applicable

Amplifier disabled and bypassed.

0001

± 1

Gain = 1

0010

± 1

Gain = 2

0011

± 1

Gain = 5

0100

± 2

Gain = 10

0101

± 2

Gain = 20

0110

± 2

Gain = 49

0111

± 2

Gain = 98

N/A

Gain = open loop for trimming. Amplifier is

enabled.

15.2.1 Programmable Gain Amplifier Settling Time

When changing channels or the gain, it’s necessary to give

the programmable gain amplifier time to settle before per-

forming an A/D conversion. This is because the input from

the previous channel could have the amplifier output near

one power supply rail and the newly selected channel or gain

may need to drive the output to the other power supply rail.

The amount of settling time is based on the gain of the

amplifier. See Table 30. It is recommended that the user wait

7.6 time constants (

τ) before performing an A/D conversion.

This should give the amplifier time to settle within 0.5 LSB of

the A/D converter. This settling time needs to be taken into

effect if either the gain is changed or if the channel is

changed. Since these values are in different registers, they

can’t be changed simultaneously and must be changed in-

dividually. The settling time starts whenever either one is

changed, but it’s not cumulative. The user should wait the

amount of settling time specified after the latter of the chan-

nel or gain change.

TABLE 30. Programmable Gain Amplifier Settling Times

GAIN

Time Constant (

τ)

Settling Time (7.6 *

τ)

Slew Rate Limited

Under-Damp Response

9µs

Under-Damp Response

6µs

0.7µs

5 µs

2µs

16µs

3.2 µs

25 µs

5.8 µs

45 µs

Open Loop

N/A

1050 µs

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