MCP4921T-ESN MICROCHIP [Microchip Technology], MCP4921T-ESN Datasheet - Page 26

• Manufacturer Part Number: MCP4921T-ESN
• Manufacturer: MICROCHIP [Microchip Technology]

MCP4921/4922
6.8
Example 6-6 illustrates a variation on a voltage follower
design where a sense resistor is used to convert the
DAC’s voltage output into a digitally-selectable current
source.
Adding the resistor network from Example 6-2 would
be advantageous in this application. The smaller R
is, the less power dissipated across it. However, this
also reduces the resolution that the current can be
controlled with. The voltage divider, or “window”, DAC
configuration would allow the range to be reduced, thus
increasing resolution around the range of interest.
When working with very small sensor voltages, plan on
eliminating the amplifier's offset error by storing the
DAC's setting under known sensor conditions.
EXAMPLE 6-6:
DS21897A-page 26
V
REF
SPI™
G = Gain select (1x or 2x)
V
D = Digital value of DAC (0 – 4096)
OUT
I
I
Building A Programmable Current
Source
b
L
=
MCP492X
=
=
3
V
I
----
V
-------------- -
R
V
L
DD
REF
sense
OUT
G
-------
2
D
12
----------- -
Digitally-Controlled Current
Source.
+
V
1
OUT
V
V
CC
CC
–
R
+
sense
I
b
LOAD
sense
I
L
6.9
The MCP492X is ideally suited for use as a multiplier/
divider in a signal chain. Common applications include:
precision programmable gain/attenuator amplifiers and
loop controls (motor feedback). The wide input range
(0V – V
in Buffered mode: the > 400 kHz bandwidth, selectible
1x/2x gain and its low power consumption give
maximum flexibility to meet the application's needs.
To configure the MCP492X in Multiplier mode, connect
the input signal to V
DAC’s input buffer, gain and output value. The DAC’s
output can utilize any of Examples 6-1 to 6-6, depend-
ing on the application requirements. Example 6-7 is an
illustration of how the DAC can operate in a motor
control feedback loop.
If the Gain Select bit is configured for 1x mode (GA = 1),
the resulting input signal will be attenuated by D/4096.
If the Gain Select bit is configured for 2x mode (GA = 0),
codes < 2048 attenuate the signal, while codes > 2048
gain the signal. V
A 12-bit DAC provides significantly more gain/attenua-
tion resolution when compared to typical Programmable
Gain Amplifiers. Adding an op amp to buffer the output,
as illustrated in Examples 6-2 to 6-6, extends the
output range and power to meet the precise needs of
the application.
EXAMPLE 6-7:
V
REF
SPI™
DD
Using Multiplier Mode
) is an Unbuffered mode and near R-R range
MCP492X
3
V
DD
OUT
= V
REF
Multiplier Mode.
 2004 Microchip Technology Inc.
Z
IN
V
FB
and serially configure the
OUT
(D/2048).
V
V
RPM_SET
RPM
V
V
CC
CC
+
–
R
+
–
sense