Digital multimeters are commonly used measuring instruments. At present, the market is currently 3.5 (three and a half) and 4.5 handheld tables for general measurement, and the other high-end is 6.5-bit desktop, the price is high, used for high precision Measurement.
With the advancement of electronic technology, high-performance low-cost devices are endless, making it possible to make a low-grade 6.5 digit multimeter, which is introduced here, which is in performance, function and cost. A design implementation scheme.
Design ideas:
Universal components for finished products: High-end DMM adopts a mixed low noise operation amplifier for a constant temperature deep to Dual JFET - Multi-slope integral high-speed high resolution ADC as a spindle measurement system, each of which The difficulty is very high, and the expensive instrument is required to debug, calibrate, so that this requirement is difficult to meet under amateur conditions, so it uses relatively low cost, reliable universal IC precision band gap, low noise Zero amplifier - 24bits low noise σΔ ADC is replaced, which can reduce component procurement difficulty, reduce overall costs, and most importantly, can get reliable performance guarantees, that is, the worst indicator indicated on Datasheet can be calculated. The overall performance of the system.
Abandon high voltage, large current range: First, high-precision measurement of these ranges is very difficult, and the input selection of the system, the protection system has put forward high requirements, the component quality requirements are high, the PCB area is large, the most important It is to be responsible for the user's personal safety. In order to avoid security issues, there is no dangerous measurement range.
Abandoning long-term stability: To ensure long-term stability by digital multi-use tables means having a high long-term stability of the entire system, the baseline should be deeply embedded, and the voltage resistor is used for precision resistance network. Wait, the cost will be significantly improved, relatively, and a standard standard (LYMEX is available) should be inexpensive, and the integral accuracy can be increased to approaching the external reference when performing comparison measurements.
Abandoning AC Measurement: Since there is no condition for the design of the AC measurement system, no.
Handheld equipment architecture: Due to the high integration of modern MCUs, the development tool is getting more simple, and the author has recently learned STM32, so it is handheld equipment.
The priority relationship between the general design elements is as follows: Low Costs> Small> Low Power> High Performance
Functions and indicators:
power supply:
3.0V ~ 6V power supply can be used in single-cell phosphate (3.2V), single-cell acid (3.7V), lithium monatomanate (3.6V), three alkaline batteries (4.5V), three nickel bad or nickel Hydrogen batteries (3.6V), power consumption 250MW (turn on data saving), 2.9V low voltage shutdown (to protect lithium electricity avoidance over discharge), software power switch, standby current. 1881.5ua.
Measurement:
1PPM resolution 1 ppm noise 5PPM linearity 1 ppm temperature coefficient.
The current measures 100mA, 10mA, 1 mA pressure drop. 1881.0.125V, 500mA fuse.
Voltage measurement of 100mV, 1V >10G high impedance input, 10V, 100V 9M low impedance input (new version of the mass purchase 1.1M resistance is a standard 10M, which is detailed later.
The resistance measurement of 100R 1K 10K 100K 1m is a constant current mode (corresponding to 1 mA, 1 mA, 100ua, 10ua, 1ua) open voltage 5V, and supports 4-wire mode.
Temperature measurement adopts a PT100 sensor, which can be processed to -200 degrees Celsius ~ 850 degrees Celsius, resolution 0.01 degrees.
In addition to temperature measurements, there are 25% over-range measurements (eg, 1V measuring 1.25V) When the automatic measurement reading is turned over, three measuring readings are turned out, and three consecutive measurement readings are increeder (. 1881. 0.11) Shift down.
System and software:
Support SD card data storage, import calibration data, update firmware from the SD card (new feature).
Support real-time clock, you can set automatic shutdown, and timing wake-up data acquisition mode.
Supports automatic range, 0-bit compensation, digital filtering.
With simple help.
Design principle
Power system:
The battery is powered, first passes through the reverse protection circuit composed of Q71, then divided into two channels, all the way HT7130 regulated to 3.0V for the MCU, and the other by the Q72 is controlled as a peripheral power supply, it is connected to two LT1372 Boost composed
The booster is rising to 15V (for ohmic current source and OLED bias) and 5V (for simulation)
The HT7130 is used herein to consider it and have extremely low static power consumption, especially suitable for the standby MCU.
The DCDC converter uses the LT1372 because of its low cost, and the device has NFB function, it can make it easy to constitute a CUK topology, a switching regulator that constitutes a negative voltage output (new version to + -15V), although due to static power consumption Large, the efficiency is low under small current output, but it is still good.
MCU system:
The MCU uses STM32F103R6T6 (in actually 101 series, but there is no retail on the market) The internal RC oscillator works on the frequency of 20 MHz (faster, and more expensive), the backup battery uses 0.22F supercapacitor, For compatibility, a 100uf tantalum capacitor pad) RTC crystal oscillate MC-306 6PF 32.768K, pay attention to the R66 to access 200K, otherwise it is easy to vain.
The MCU uses the 5-wire SWD port for flash burning and debugging.
PA0 constitutes a software power switch that can wake up the MCU from the standby mode.
OLED uses a serial mode, 4x3 matrix keyboard, and SD card use SPI mode.
Other ports are used to control the simulation board.
Enter the selection:
First watch voltage - resistance section
Relay K1 Select to access the HI input terminal directly into an analog switch or perform partial pressure.
Relay K2 Select to inject ohmic current sources into Hi or Low.
Both relays above use magnetic retaining types to avoid long-term power generation caused by thermal potential errors.
HS, LS, and Hi input through R01-R06 and R17-R1A D17-D16 protection circuits are limited to + -2.0V, and then plus from 9 string: 11 and 1.1M spur pressure arrays 99: 1 points The voltage of the pressure is taken into 8 choice analog switch.
The current portion passes through a 500mA fuse-full bridge protection circuit, and the shunt resistor for the access 100R 10R or 1R is selected through the relay, and the pressure drop on which is also sent to the analog switch, and the following follower constituted by the U01A will increase the diode bridge. The potential of the midpoint is to the current input terminal potential, thereby reducing their leakage current.
The U01B provides a midpoint reference potential (that is, the potential of the LOW terminal) for the measurement system.
It is desirable here that the partial voltage resistance is a 50 ppm wafer resistor constituting an array. The following PCB is applied to copper, from the actual effect, it is very nice.
U01 uses low cost MCP6002 due to high requirements.
The simulation switch originally plans to use MAX328, but due to the source reasons, use the ADG508 instead.
Program control zoom:
The AD8629 is used as the main amplifier, which is a low noise chopped zero amplifier, and the U20A is configured to X1 or X10 amplifier based on the feedback signal selected by the U27, and the U20B is a buffer that drives the driving capacitive load for driving the ADC.
The same one of the same pressure resistors is 3 string: 3 and the 9: 1 divider, due to the high-speed CMOS 74HC4053 due to the high-speed MTR
REF AND ADC:
The U44 is a 2.5V precision band gap basis, which uses the ADR421B he can steadily drive 1UF capacitive load, the maximum 3 ppm / c temperature coefficient, 1.5UV PP 0.1 ~ 10 Hz noise
The U43 uses 24bit low noise σΔ ADC LTC 2440 to provide a valid bit close to 21bits.
The U41 is a low noise LDO regulator LP2985 to provide a 5V power supply to the analog section.
Ohm current source:
U31 is a low-bias current precision op amp, where U31A and Q31 functions as a reference voltage scaled-transfer, converting 2.5V reference into a standard resistor (RJ31 ~ RJ34) common low 1V to U31B, The analog switch is used to select 4 standard resistors (Kairen pickup), Q32 is PJFET is controlled by U31B to maintain the voltage of 1V, Q33 ~ Q35 and D31 constitute a protection circuit (Q34, Q35 actually high back pressure PNP pipe 2N6520 Q33 is a low leakage current N-JFET PN1117A).
The DZ1 and R30 constitute a step-down circuit such that the output of the U31B is sufficient to turn the Q32.
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