Principle and Application of High-Performance ∑-Δ ADC

1 Overview

MAX1403 is an 18-bit, over-sampling ADC chip that uses a ∑-Δ modulator and a digital filter to achieve true 16-bit conversion accuracy. In applications, in order to obtain a high output data rate, the digital filter factor can be selected and the conversion resolution can be reduced. The sampling frequency of the modulator can be used as the preferred condition for the minimum power consumption and the highest output data rate.

MAX1403 can provide three true differential input channels with independent programming (gain from 1V/V to +128V/V) and can compensate for the DC offset of the input parameter voltage. These three true differential input channels can also form five pseudo differential input channels. In addition, the chip also has two additional differential correction channels, which can correct gain and offset errors.

MAX1403 can process all input signals and provide conversion results to the outside through the serial digital interface. When the host clock frequency is 2.4576MHz or 1.024MHz, the on-chip digital filter can process the line frequency and related harmonic frequencies and make the amplitude of these frequencies zero. This can achieve better filtering effects without the need for an external filter, and at the same time, it also helps to improve the quality of the digital signal at the output.

The main features of the MAX1403 are as follows:

●18-bit resolution;

●Has 8 registers;

●Low power consumption

●Has two matched sensor excitation current sources;

●3 true differential inputs or 5 pseudo differential input channels;

●2 additional input correction channels;

●With a bidirectional serial communication interface;

●Analog power supply and digital power supply adopt independent power supply mode;

●Gain and offset can be controlled by software.

2 Pin Function

The MAX1403 chip uses a 28-pin SSOP package, and its pin arrangement is shown in Figure 1. The functions of each pin are as follows:

CLKIN: clock input pin;

CLKOUT: Clock output pin. When using an external crystal oscillator, connect the external crystal oscillator between CLKIN and CLKOUT; when using other external clock signals, the clock signal (frequency is 2.4576MHz or 1.024MHz) is input at CLKIN, and CLKOUT is not connected.

CS: Chip select input pin. Low level is effective. When CS is low, the chip is allowed to work in three-wire interface mode and can select multiple devices on the serial interface or serve as a frame synchronization signal.

RESET: Reset input pin. Low level is valid. When RESET is low level, the control logic, interface logic, digital filter and analog modulator can be reset after power-on; when RESET is high level, the reset is exited.

DS1: Auxiliary digital input bit 1 digital input pin;

DS0: digital input pin for auxiliary data input bit 0;

OUT2: sensor excitation current source 2;

OUT1: sensor excitation current source 1;

AGND: Analog ground. It is the reference point of the analog circuit;

V+: Analog positive power supply voltage input pin, the selection range is +2.7V~+3.6V;

AIN1~AIN6: analog input channel 1~6 pins respectively;

CALGAIN-: Gain correction negative input pin;

CALGAIN+: Gain correction positive input pin;

REFIN-: Differential reference negative input pin;

REFIN+: differential reference positive input pin;

CALOFF-: offset correction negative input pin;

CALOFF+: Offset correction positive input pin;

DGND: Digital ground pin. It is the reference point of digital circuit;

VDD: digital power supply voltage input pin. The range is between +2.7V and +3.6V;

INT: interrupt output pin;

DOUT: serial data output pin;

DIN: serial data input pin;

SCLK: serial clock input pin;

[page] 3 Internal structure

The internal functional structure of MAX1403 is shown in Figure 2. As can be seen from the figure, the chip consists of a switch structure, a modulator, a PGA (programmable gain amplifier), two buffers, a DAC, a digital filter, an oscillator, two matched sensor excitation current sources and a bidirectional serial communication interface.

4 Main parameters

In order to fully utilize the performance of MAX1403 and use it correctly, it is necessary to have a quantitative understanding of the recommended parameters and limit parameters. The main parameters are described as follows:

4.1 Working parameters

The recommended operating parameters for the MAX1403 are as follows:

●Analog power supply voltage (V+): 2.7V～3.6V;

●Digital power supply voltage (VDD): 2.7V～3.6V;

●Reference voltage: 1.25V;

●Clock frequency: 2.4576MHz;

●No missing code accuracy: 16 bits;

●Analog input voltage: (VAGND-30mV) ~ (V++30mV);

●Digital input voltage: 0.4V～2V;

●Digital output voltage: 0.4V～(VDD-0.3V);

●Operating temperature:

MAX1403CA1：0～+70℃；

MAX1403EA1：-40～+85℃ ；

●Power consumption: 2~22mW;

4.2 Limit parameters

Below are the limit parameters of the MAX1403 ADC chip.

●Analog power supply voltage (V+): -0.3V～+6V;

●Digital power supply voltage (VDD): -0.3V～+6V;

●The voltage between analog ground and digital ground: -0.3V～+0.3V;

●Analog input voltage: -0.3V～(V++0.3V);

●Analog output voltage: -0.3V～(V++0.3V);

●Reference voltage: -0.3V～(V++0.3V);

●All digital output voltage: -0.3V～(VDD+0.3V);

●All other digital input voltages: -0.3V to +6V;

●Clock input and clock output voltage: -0.3V～(VDD+0.3V);

●Power consumption: 50mW.

5 Application Circuit

Because MAX1403 has multiple functions, it is very popular in various microcontroller systems with wide dynamic range (electronic scales and pressure sensors) and serial interfaces. Several main application circuits are given below.

5.1 RTD Application Circuit

The practical circuit of 3-wire RTD composed of MAX1403 and a few peripheral components is shown in Figure 3. The two current sources (200μA) in the figure are strictly matched, and their purpose is to compensate for the error in the 3-wire RTD circuit. In the 3-wire RTD circuit, if only one current source is used, the lead resistance will cause error to the system. At this time, the 200μA current through RL1 will generate an error voltage and add it to the two upper input terminals (AIN1 and AIN2) of the PGA. If another current source with the same size as the previous current source is used. Then the current source will also generate an error voltage in RL2, which is the same size as the error voltage of RL1 and opposite in direction, so as to ensure that the error voltage at the input terminals of AIN1 and AIN2 is zero, that is, it is not affected by the lead resistance. The reference voltage in Figure 3 is provided by the voltage drop of a current source (200μA) in a 12.5kΩ resistor. This setting can ensure that the ADC obtains a more accurate ratio result.

[page] The 4-wire RTD application circuit is shown in Figure 4. The only difference between this figure and the 3-wire RTD circuit is that there is no error voltage generated by the lead resistance at the measurement input terminals AIN1 and AIN2. The current source OUT1 can provide an excitation current to the RTD, and the current provided by the current source OUT2 can generate a reference voltage for the modulator to use at the resistor RREF. In the 4-wire RTD application circuit, the RTD temperature error in the analog input voltage is caused by the temperature drift of the RTD current source, which can be compensated by changing the reference voltage, so that the error voltage at the input terminals AIN1 and AIN2 reaches zero.

5.2 Interface circuit with microcontroller

The actual circuit of the interface composed of MAX1403 and MCU 68HC11 is shown in Figure 5. As can be seen from Figure 5, the interface circuit is very simple and is one of the few I/O ports of Huagang MCU. When the MCU has a hardware SPI (Serial Peripheral Interface), a three-wire interface can be used and directly connected to MAX1403. The SPI hardware generates 8 pulses on SCLK to shift data in on one pin and out on another pin.

For best results, use a hardware interrupt to monitor the INT pin and acquire new data when the hardware interrupt is active. If the hardware interrupt is inactive or the interrupt execution time is longer than the selected conversion rate, use the SYNC bit to prevent data from being read out of the data output register while the measurement is in progress.

Another interface circuit of MAX1403 is shown in Figure 6. As can be seen from the figure, this interface circuit consumes fewer microcontroller I/O ports and has a simpler circuit. All microcontroller I/O pins can be interfaced with MAX1403. If a bidirectional open-drain I/O pin is valid, DOUT and DIN can be connected to further reduce the number of interface pins. To use the MAX1403 three-wire interface, the CS pin in the figure must be grounded.

5.3 4～20mA transmitter

The 4-20mA transmitter composed of MAX1403, μC/μP and DAC circuits is shown in Figure 7. This is a low-voltage, single-power-supply transmitter that is easy to interface with an optocoupler and has very good performance. The transmitter obtains power (energy) from the 4-20mA loop, so that the current of the transmitter circuit is limited to 4mA. If the loop current tolerance is further limited to 3.5mA, the transmitter can still maintain the loop current at 3.25mA because the MAX1403 itself consumes 250μA (0.25mA).

6 Problems in printed circuit board and component assembly

In order to achieve the best performance of the ADC, a printed circuit board with separate analog and digital grounds must be used. In the design of the printed circuit board, special attention should be paid to the layout of the ground line. Usually, the analog ground and digital ground are set independently in their respective circuits, and then the analog ground and are connected to one point (star mark). If there is only one MAX1403 in the system, the AGND and DGND pins of the chip can be connected to the ground plane together; if there are multiple MAX1403s in the system, the AGND and DGND pins of multiple chips can be connected, and then connected to a common point, and this common point should be as close as possible to the star ground of the MAX1403. It is strictly forbidden to design the digital ground under the chip, because this will couple noise to the chip, thereby affecting the normal operation of the ADC. However, the analog ground should be run under the chip, because this can reduce the coupling of digital noise. The input line of the power pin of the MAX1403 should be as wide as possible to provide a low impedance channel, thereby reducing the impact of pulses on the power line.

Since MAX1403 is a high-resolution ADC, the coupling circuit of the power supply is particularly important. Therefore, when designing the printed circuit board, a decoupling circuit should be added to all analog power inputs, that is, a 10μF lithium capacitor and a 0.1μF ceramic capacitor should be connected in parallel to the ground. The components of these decoupling circuits should be as close to the power pins of the chip as possible, so as to obtain a better decoupling effect and eliminate the interference caused by the lead crossing.