X9241高精度数字电位器的使用

VL VH
PLeabharlann BaiduT 0
5+
1/2 LT1112
6-
7
VW
POT 1/2 (POTs cascaded)
VLcasc
VL
Figure 3
Due to wiper MOSFET loading and variations in the resistive elements of each pot, a piecewise linear behavior appears when transitioning between voltage intervals. By testing a circuit with a sample X9241U (49.37KΩ, 49.38KΩ, 49.32KΩ, 49.24KΩ) device, the representative resolutions between adjacent taps are found and shown in Fig. 4. The use of low offset unity gain buffers as shown in Fig. 3, as well as X9241 devices with both POT1 and POT2 well-matched can improve the linearity at the expense of monotonicity. To guarantee monotonicity for any X9241 device at the expense of resolution, software range transitions could be adjusted to occur between tap 127 and tap x (x > 0). The finer resolution within each voltage interval would provide sufficient precision, however connecting another pot as a rheostat between VH and the VH terminals of POT0 and POT3 will add a variable voltage drop for "trimming" the output of VW even further and would be an alternate method to the use of buffers.
6581-1.4 11/20/95 GHCIV
Xicor, Inc. • 1511 Buckeye Drive • Milpitas, CA 95035 • (408) 432-8888
AN43-1
Xicor Application Note
AN43
VH
POT 3 3
+ 2-
1/2 LT1112 1 VHcasc
In some applications (e.g. dynamic biasing of LASER diodes), the use of digital potentiometers or trimDACs to control voltage levels is limited by resolution, accuracy, or interface difficulties. Using the simple, but not so obvious, Vernier scheme in Fig. 1, which consists of an X9241 quad E2POT and a little bit of software, an 8001-tap position nonvolatile digital pot can easily be implemented that exhibits a typical tapto-tap resolution of 0.008%.
Tap Implementation (x=1), but without LT1112 Buffers.
6581-1.4 11/20/95 GHCIV
AN43-2
Xicor Application Note
AN43
/**************************************************************************/
/* specific and can be extracted from the Xicor BBS (800-258-8864).
/* For example, the required low level routines may as simple as:
/*
/*
start(), stop(), SDA_high(), SDA_low(), SCL_high(),
/*
SCL_low(), ack(), nack(), send_byte(), & get_byte()
/*
/* Since C code exists on the Xicor BBS to drive the X9241, only those
/* functions relating to this Vernier scheme are included here. This
/*
/*
eepot_8000_read()
returns integer value between 0 and 8000
/*
corresponding to current wiper position
/*
eepot_8000_write(n)
changes wiper position to that specified by n
/*
/* The following routines implement a high resolution EEPOT with the X9241.
/* This C code can be adapted to any system since it is written at a
/* high level. The low level I2C routines will be processor/controller
/* code was written and debugged using Borland Turbo C. To use these
/* routines, just include them into a larger program and call any of
/* these five routines as desired.
Figure 4 - Wiper Voltage Per Tap Position as a Percentage of Total Applied (VH-VL). Inset Graph Shows
Resolution Between Adjacent Taps as a Percentage of Total Voltage Applied. Measurements Taken with 7940
100
1
80
0.5
0
0.5
60 1 0 2000 4000 6000 8000
TAP INTERVAL
40
20
0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 TAP POSITION
Figure 2
POT0 and POT3 set the voltage interval for POT1/2. As the wiper position of POT1/2 moves up and down, the wiper positions of POT0 and POT3 adjust when necessary. If the wiper of POT1/2 is incremented beyond tap 127, then the wipers of POT0 and POT3 will be incremented and the wiper of POT1/2 will be returned to tap 0. Likewise for decrements of the wiper of POT1/2 below tap 0, the wipers of POT0 and POT3 will be decremented and POT1/2 will be set to tap 127. The X9241 is nicely suited for this implementation since tap positions can be directly changed in software without the need to transition through each intermediate position.
Application Note
AN43
Software Implements a High Resolution Nonvolatile Digital Potentiometer
by Gray Creager, October 1994 (from a design idea published in EDN magazine (June 8, 1995))
should be another redundant tap position when transitioning between tap 127 of a particular voltage interval and tap 0 of the next voltage interval, however these tap positions are not actually redundant and their use can improve the linearity. With 127 distinct outputs for each of 63 distinct intervals, there are 8001 distinct VW outputs available between VH and VL. Fig. 2 illustrates the concept behind this scheme.
Figure 1
The X9241 includes four separate 64-tap nonvolatile E2POTs with a mechanism to serially cascade adjacent POTs, allowing up to 253 distinct tap positions. However if only two POTs are cascaded together and the remaining two POTs are used to set the levels of VHcasc and VLcasc, then a much higher degree of resolution can be attained. Since the wipers of the POTs that set VHcasc and VLcasc must always be 1 position apart, there will be 63 distinct voltage intervals applied to the remaining cascaded POTs. This software neglects one redundant tap position per interval when VW is set to tap 64. Ideally, there