cop8ame9 National Semiconductor Corporation, cop8ame9 Datasheet - Page 52

• Manufacturer Part Number: cop8ame9
• Description: 8-bit Cmos Flash Microcontroller With 8k Memory, Dual Op Amps, Virtual Eeprom, Temperature Sensor, 10-bit A/d And Brownout Reset
• Manufacturer: National Semiconductor Corporation

www.national.com
15.0 A/D Converter
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.
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.
TRIM
Bit 7
Bit 7
Bit 9
Bit 1
0
0
0
0
0
0
0
0
1
Bit 6
Bit 6
Bit 8
Bit 0
Open Loop
GAIN2
GAIN
10
20
49
98
Bit 5
Bit 5
Bit 7
1
2
5
0
0
0
0
1
1
1
1
X
0
TABLE 26. ADRSTH
TABLE 27. ADRSTL
Bit 4
Bit 4
Bit 6
0
GAIN1
X
Bit 3
Bit 3
0
0
1
1
0
0
1
1
Bit 5
0
TABLE 30. Programmable Gain Amplifier Settling Times
(Continued)
Bit 2
Bit 2
Bit 4
0
GAIN0
X
0
1
0
1
0
1
0
1
TABLE 29. Gain Bit Assignments
Bit 1
Bit 1
Bit 3
Under-Damp Response
Under-Damp Response
0
Time Constant (τ)
Slew Rate Limited
Bit 0
Bit 0
Bit 2
3.2 µs
5.8 µs
0
0.7µs
2 µs
N/A
Gain Tolerance
Not Applicable
52
N/A
±
±
±
±
±
±
±
ENAMP1 Enables stand-alone amplifier (AMP1) to port B.
ENTS
TRIM
GAIN2:0
Reserved These bits are reserved and must be 0.
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.
1
1
1
2
2
2
2
AMP1
Bit 7
EN-
ENTS
Bit 6
Enabled = 1. Disabled =0.
Enable internal temperature sensor. Enabled =
1. Disabled = 0.
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.
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
TRIM
Amplifier disabled and bypassed.
Gain = 1
Gain = 2
Gain = 5
Gain = 10
Gain = 20
Gain = 49
Gain = 98
Gain = open loop for trimming. Amplifier is
enabled.
Bit 5
TABLE 28. ADGAIN
Bit 4
Settling Time (7.6 * τ)
Reserved
Slew Rate Limited
Bit 3
1050 µs
16 µs
25 µs
45 µs
5 µs
9µs
6µs
GAIN2 GAIN1 GAIN0
Bit 2
Bit 1
CC
≥ 4.5V.
Bit 0