LTD2261:High-precision, Multi-channel, 40-kSPS, 24-bit, Delta-sigma ADCs

The LTD226X family are high-precision, multi-channel, 40-kSPS, delta-sigma (Δ-Σ) analog-to-digital converters (ADCs). Features, for example, programmable gain amplifier (PGA), precise and low-drift voltage reference, monitors for fault detection, general purpose input output options are included in this highly-integrated chip making it a single-chip solution to deal with measurements tasks such as weigh scales, resistance temperature detectors (RTDs) and thermalcouple sensors.

The main path of analog-to-digital conversion consists of an input multiplexer (5 channels for LTD2260 and 10 channels for LTD2261), a low-noise PGA, a 24-bit Δ-Σ modulator, a precise and low-drift voltage reference, and a programmable digital filter. The 1 GΩ high input impedance of the PGA minimizes the error induced by sensor loading. The device provide two individual current source with programmable supplying current, that simplifies the circuit of RTDs measurements. 

The conversion is completed by a fourth-order Δ-Σ modulator combined with a flexible digital filter. The filter supports a simultaneous 50-Hz and 60-Hz rejection mode and single-cycle settling mode. Associated with internal signal and reference monitors, a temperature sensor and CRC data verification, the output data is more reliable. The LTD226X family are available in 5 mm × 5 mm QFN packages and specified in temperature range from -40℃ to 125℃.

Features and Benefits

• High-precision, 24-bit delta-sigma ADC

• Low noise: 30 nVRMS (@20 SPS, gain=128)

• Linearity: 2 ppm

• 5 single-ended inputs or 3 differential inputs (LTD2260)

• 10 single-ended inputs or 5 differential inputs (LTD2261)

• PGA gain: 1 to 128

• Input voltage range: ±7 mV to ±5 V

• Data rate: 2.5 SPS to 40 kSPS

• Low drift 2.5 V reference: 2 ppm/℃

• Simultaneous 50-Hz and 60-Hz rejection mode

• Single-cycle settling mode

• PGA voltages and reference monitors

• 5 V or ±2.5 V supply

• Integrated temperature sensor

• Cyclic redundancy check

• Two individual current source

• Sensor burn-out detection

• 4 GPIOs available

• AC excitation for bridge sensors

• SPI compatible

Applications

• Temperature and pressure measurement

• Analog input modules in PLC and DCS

• Weigh scales and strain-gauge digitizers

• Scientific and lab instruments

Typical Applications

Three wire resistance temperature sensor

1. Advantage

   Using two high-precision independent current sources in LTD2261 chip, single-chip high-precision temperature measurement is realized

   The metal wire parasitic resistance voltage drop compensation significantly reduces the system measurement error caused by the parasitic effect

   Proportional measurement scheme cancels out current derived from noise and temperature drift

2. Calculation process

   
   

1. Analog input A_IN1 and A_IN2 voltages:

   V_AIN1 = [I_IDAC1 * (R_RTD + R_LEAD1)] + [(I_IDAC1 + I_IDAC2) * (R_LEAD3 + R_BIAS)]

   V_AIN2 = (I_IDAC2 * R_LEAD2) + [(I_IDAC1 + I_IDAC2) * (R_LEAD3 + R_BIAS)]

2. Differential input voltage：

   V_AIN1 - V_AIN2 = [I_IDAC1 * (R_RTD + R_LEAD1)] - (I_DAC2 * R_LEAD2)

3. The LTD2261 current source mismatch is only 0.1%，I_IDAC1 = I_IDAC2 = I_IDAC，R_{LEAD1 =} R_LEAD2，so:

   V_AIN1 - V_AIN2 = I_IDAC * R_RTD
   

   V_REF = I_IDAC * R_REF

4. Output code value=2²³ * Gain * (V_AIN1 - V_AIN2) / V_REF = 2²³ * Gain * R_RTD / R_REF