Eight-channel 24-bit micropower no-delay delta-sigma analog-to-digital conversion LTC2408

    Abstract: LTC2408 is a Δ-Σ analog-to-digital converter developed by the American LINEAR company with the characteristics of low noise, low co-consumption, and high speed. It can directly receive input signals from sensors and is suitable for measuring low-frequency signals with a large dynamic range. It can be widely used in pressure measurement, direct temperature measurement, gas analysis and other fields. This article introduces the working principle and application circuit of LTC2408.

1 Overview

    LTC2408 is a Δ-Σ analog-to-digital converter developed by the American LINEAR company with low noise, low power consumption, high speed and other characteristics. It uses Δ-Σ technology to further reduce the impact of the noise environment, thus becoming an industrial and process control Ideal for applications. In addition, using the LTC2408 in the system allows system designers to achieve very high resolution because the noise performance of the LTC2408 is much better than that of the integrating analog-to-digital converter. It can directly receive input signals from sensors and is suitable for measuring low-frequency signals with a large dynamic range. It can be widely used in pressure measurement, direct temperature measurement, gas analysis and other fields. The LTC2408's 4-wire serial interface is compatible with SPI and MICROWIRETM standards and is easy to interface with microprocessors or digital signal processors. It can be widely used in data acquisition systems and programmable logic control systems.

    The LTC2408 has the following features:

    ●Contains a 24-bit resolution ADC and a multiplexer with 8 analog input channels;

    ●Single clock cycle settling time simplifies multiplexer operation;

    ●Nonlinear error is 4PPM, no bit errors;

    ●Full-scale error is 4PPM;

    ●The imbalance is 0.5PPM;

    ●0.3PPM noise;

    ●Built-in oscillator, no external clock components required;

    ●50Hz/60Hz notch filter, minimum attenuation 110dB;

    ●The reference input voltage range is 0.1~Vcc;

    ●The active zero level can expand the input voltage range to -12.5% ​​VREF ~ 112.5% ​​VREF);

    ●Works with 2.7V～5.5V single power supply

    ●Features low supply current (20μA) and automatic shutdown mode.

2 Working principle and packaging

    The internal structure and functional block diagram of LTC2408 is shown in Figure 1. It contains 8 analog channels of MUX and Δ-ΣADC and clock oscillator, so there is no need for external clock components. The first conversion after the analog channel is changed will be effective. By setting the pins of LTC2408, a notch can be formed at a frequency of 50Hz or 60Hz+2%, with an attenuation of up to 110dB. When driven by an external oscillator, the notch frequency can be selected within the range of 1Hz to 120Hz.

    The reference input voltage of LTC2408 is 0.1V~Vcc. Since the LTC2408 can expand the input range to -12.5% ​​VREF ~ 112.5% ​​VREF, it can well eliminate offset and over-range problems caused by pre-sensors or signal conditioning circuits.

    LTC2408 is packaged in a 28-pin plastic SSOP, and its pin arrangement is shown in Figure 2. The functions of each pin are as follows:

    GND (pins 1, 5, 16, 18, 22, 27, 28): Ground. Analog circuits, digital circuits and reference voltages use a common reference ground. It must be connected directly to the ground plane or at a single point through the shortest lead.

    Vcc (pins 2, 8): Positive power supply. A 10μF solid tantalum capacitor and a 0.1μF ceramic capacitor should be connected in parallel between Vcc and GND for bypass. And the capacitor leads should be as short as possible.

    VREF (pin 3) reference voltage input. The reference voltage range is 0.1V~Vcc.

    ADCIN (Pin 4): Analog input. The analog input voltage range is -0.125VREF~1.125VREF. If VREF>2.5V, the input analog voltage needs to be limited to the allowed input voltage range of the pin (-0.3V to Vcc+0.3V).

    COM (Pin 6): Reference ground for signal voltage. This end must be connected directly to the ground plane via the shortest lead.

    MUXOUT (Pin 7): The output voltage terminal of MUX is the output voltage pin of the multiplexer. Connected to ADCIN during normal operation.

    CH0~7 (pins 9~15 and 17): Analog input channels 0~7 of the multiplexer.

    CLK, SCK (pins 19, 25): conversion clock input. Used to control the status of synchronous transmission of series data to MUX and ADC. When the ADC signal drops from high level to low level, this signal must be low level.

    CS MUX (Pin 20): Chip select input of MUX. When CS MUX is high level, MUX receives the address of an analog channel. When CS MUX is low level, MUX is gated and the selected channel is connected to MUXOUT, and the signal of the channel is A/D converted at the same time. Under normal circumstances, CS MUX and CS ADC are connected in parallel and driven at the same time.

    DIN (Pin 21): Digital signal input terminal. Enter the address of the multiplexer from DIN.

    CS ADC (pin 23): digital input, active low. When CS ADC is low level, SDO outputs data; otherwise, SDO does not output data. As each conversion ends, the ADC automatically enters sleep mode. As long as CS is high, the ADC remains in this low power state. CS can only wake up the ADC when it is low. When CS rises from low level to high level, the writing operation is suspended and a new conversion is started.

    SDO (Pin 24): Three-state data output terminal, serial data output during write operation. When the chip select CS is high (CS=Vcc), SDO is in a high impedance state. When the ADC is in the conversion and sleep period, SDO can be used as the conversion status output terminal, so that CS falls to low level to read the conversion status.

    F0 (Pin 26): Digital input, used to control the center frequency and conversion time of the ADC's notch filter. When F0 is connected to Vcc (F0=Vcc), the converter uses the internal clock oscillator and the first zero of the digital filter is 50Hz. When F0 is connected to FND (F00V), each zero point of the digital filter in the converter is 60Hz. When R0 is driven by an external clock signal with frequency fEOSC, the converter uses this signal as its clock signal and the first zero of the digital filter is fEOSC/2560Hz.

3 Application design

    Figure 3 shows a typical conversion timing diagram. If CS is connected to the CS ADC and CS MUX, then it is high when the MUX is strobe. CLK transmits data to MUX as the basis for analog channel selection. On the rising edge of CLK, data is transferred via DIN. Different numbers correspond to different analog channels, and their truth tables are listed in Table 1. The multiplexer cannot write until CS falls low and after the previous conversion has completed. In order to ensure that the previous conversion has been completed, after reading the output data, the multiplexer should be delayed at least tconv (about 135ms) before performing the write operation to ensure correct operation. At the falling edge of CS, CLK should at low level.

Table 1 Analog channel selection truth table

    When the multiplexer is writing, the ADC is in a low-power sleepy state. Once the write operation of MUX is completed, the data of the last conversion is read. As the data is read, the analog input voltage is connected to the new selected channel to start a new conversion cycle.

    Under the control of CLK, data is output from SDO. Data is latched on the rising edge of CLK. After 32 clock cycles, SDO goes high, which means a new conversion has begun. If CS is still low, the last analog channel of the multiplexer is still selected. As mentioned above, after CS rises to high level and delays tconv, the analog channel can be reselected by inputting data into DIN. Because the LTC2408's settling time is a single clock cycle, any analog channel can be selected after each conversion. There is no delay in converting the result once. Regardless of which analog channel is selected, each conversion is independent of the previous conversion.

4 Digital measurement circuit for multiple physical quantities

    Figure 4 shows a multi-functional measurement circuit designed with LTC2408 to digitally measure various physical quantities in nature. Single-ended signal conditioning circuits are used here. Although it is better to use a differential signal input method when the bridge sensor operates in a high-noise environment or at a distance from the ADC device, the low power consumption of the LTC2408 allows the circuit to operate very close to the sensor. Therefore, using a single-ended signal conditioning input can greatly simplify the sensor output method. When differential signals are required, chopper or self-calibration circuits can be used to match the LTC2408.

    In Figure 4, through a resistor network connected to Channel 0, the LTC2408 is capable of measuring DC voltages from 1mV to 1kV without the need for autoranging. The power of the 990kΩ resistor in the resistor network should be 1W and capable of operating at high voltage. This resistor can also be replaced by a series of lower-priced, smaller-power metal film resistors in series.

    Connected to channel 1 is the LT1793FET input op amp, which acts as an electrometer amplifier at high impedance and low frequency and can be used to measure the pH value of the solution. The gain of this circuit is 21, and the input signal range is -15nV~250mV. Since sensors with high output impedance cannot directly drive capacitive switching ADCs, there must be an amplification link in the circuit. The LT1793 was chosen because of its low input bias current (10pA maximum) and low signal-to-noise ratio. It is recommended to use a guard ring in high-impedance sensors. Otherwise, due to the influence of circuit board leakage current, the measured results will be biased, thereby reducing the measurement accuracy.

    Connected to Channel 2 is a precision half-wave rectifier circuit using the LTC2408's internal delta-sigma ADC as the integrator. This circuit can be used to measure frequencies from 60Hz, 120Hz or 400Hz to 1kHz with excellent results. The LTC2408's internal sine sampling filter effectively filters out any frequency within the above range. When the frequency is higher than 1kHz, the measurement level of the circuit will be reduced due to the limitation of the amplifier gain bandwidth and the combined influence of the transient process. The dynamic range of the circuit is limited by the input bias voltage of the op amp and the overall noise of the system. Matching a chopper-zero-stabilized operational amplifier LTC1050 (Vos=5μV) can expand the dynamic range by about 5 orders of magnitude. R6 and R7 are composed of a precision three-terminal dual 10kΩ resistor network to maintain gain and temperature stability. In most applications a maximum resistor tolerance of 0.1% and a maximum temperature coefficient of 5ppm/°C are allowed.

    The circuit formed by connecting channel 3 and channel 4 of the LTC2408 to a three-wire 100Ω platinum thermistor (PT RTD) can measure the RMS/RF signal power with frequencies ranging from tens of Hz to 1GHz. The characteristic of this circuit is that the signal energy can be measured by the 100Ω RTD when it is dissipated as heat at the 50Ω resistor terminal. The lead resistance of the RTD can be compensated by the readings of the two channels. The result obtained by multiplying the reading of channel 4 by 2 and subtracting the reading of channel 3 is the accurate value of the RTD.

    The thermistor connected in the form of a half-bridge in the circuit is connected to channel 5 of the LTC2408, and the temperature of the box can be measured using the RTD thermal energy measurement method as described above. In general, thermistors have very high resolution within the specified temperature range, and it is possible to achieve a measurement resolution of 0.001°C. However, the effect of self-heating of the thermistor and its deviation and the thermal conductive structure of the circuit limit the acquisition of high resolution. With the half-bridge circuit shown, the temperature measurement range of the LTC2408 will be expanded by 5 times.

    The infrared thermocouple thermometer is connected to channel 6 of the LTC2408 and can be used for non-contact temperature measurement. Assuming that the noise of the LTC2408 is 0.3PMRMS, the measurement resolution using an infrared thermocouple thermometer is approximately 0.03°C, which is equivalent to the conventional J-type thermocouple thermometer. Because the infrared thermocouple thermometer is self-calibrating, it does not require any external cold junction compensation; it does not require the use of traditional open thermocouple detection circuits; and it has an output impedance of approximately 3kΩ. Correspondingly, a traditional thermocouple thermometer can also be connected directly to the LTC2408 (not shown in Figure 4), with cold junction compensation provided by an external temperature sensor connected to a different channel, or the LT1025 can be used for single cold junction compensation.

    The photodiode connected to channel 7 senses sunlight with a current resolution of 300pA. The photodiode in Figure 4 is in photoconductive mode. The LTC2408 is available in both photoconductive and photovoltaic modes. When a photodiode (Hamnatsu S1336-5BK) with a light intensity of 500mA per watt is selected, its output depends on two factors: the effective detection area (2.4mm×2.4mm) and the light intensity. Using a 5kΩ resistor increases the measurable light intensity to 368W/m2 at a wavelength of 960nm (the light intensity of direct sunlight is approximately 1000W/m2). Because the resolution is 300pA, the measurable light intensity variation range is 6 orders of magnitude.

5 Conclusion

    The multi-input conversion and high resolution characteristics of the LTC2408 device enable it to have an extremely wide range of applications. The application circuit introduced in this article demonstrates its flexible matching capabilities. With only a small amount of peripheral circuitry, the LTC2408 can measure very weak signals over a wide range. Therefore, in weak signal detection and various industrial measurement and control systems, the advantages of LTC2408's high precision and high resolution will be fully reflected.