In many cases, selecting a suitable ADC in the circuit can greatly simplify the front-end circuit. Here we look at an example of a resistive bridge:
An instrumentation amplifier and an operational amplifier are used here, and they actually mainly perform three functions:
1. Suppress the 2.5V common-mode signal;
2. Amplify the -12.44mV differential-mode signal by 110 times, thereby It meets the requirements of ADC sampling accuracy;
3. Although the signals output by the bridge are all unipolar (positive level), the differential mode signal -12.44mV to 12.56mV is bipolar. The op amp circuit adds a DC bias of 2.048V to the bipolar signal, making maximum use of the dynamic range of the 0-4.096V input ADC; so
if we can find an ADC that has a fully differential input (providing a complete Common-mode rejection capability), and with ultra-high precision, we may not need INA for common-mode rejection and level-up, and can complete the requirement of obtaining 2000 readings in 25mV without two stages of amplification, that is to say, the ADC’s The minimum resolution is less than 12.5uV!
(1) Use △-∑ ADC to complete the work of the entire signal chain
In the resistance bridge, when the resistance value of the variable resistor R3 changes from 9.9k to 10.1k, the output differential mode signal is 12.56mV to -12.44mV, that is to say, the resistance changes in 0.2k ohms During the process, the detectable signal variation range is 25mV. If the desired accuracy is 0.1 ohms, that would take 2000 readings, that would take 2000 readings in the 25mV range, that would be 12.5uV each.
Taking ADS1232 as an example, it is a 24-bit 5V full-scale input ADC. It can be seen from the table below that when its data throughput rate is 10SPS and the internal PGA is set to 1, the peak-to-peak noise of its input stage is only 1.79uV.
At this time, the noise-free number of bits of ADS1232 can reach 21.4 bits. Here we take 2^20 to be approximately 10^6 to calculate the size of each LSB of ADS1232: 1LSB=5V/10^6=5uV < 12.5uV. 5000 readings can be obtained in the full-scale differential mode output of the bridge at 25mV, which is more than satisfactory. Therefore, the circuit can be simplified as:
Although in theory we can use ADS1232 to complete the design, but in system design, it is very difficult to control the peak-to-peak value of system noise (including device noise, radiation noise and conduction noise, etc.) to be less than 12.5uV. At this time, we can use the internal PGA of ADS1232 to amplify the input differential mode signal (optional multiples are 1, 2, 4...128), for example, we amplify the input 25mV differential mode signal by 64 times through the internal PGA to obtain 1.6V The useful differential mode signal, our system noise only needs to be less than 1.6V/2000=800uV to complete the work, which reduces the requirements for system design a lot, especially after omitting a large number of operational amplifiers and resistors, the noise Source reduction is more conducive to reducing system noise. From the chip data sheet, we can see that after the PGA multiple is increased, the input stage noise of the ADS1232 becomes: 125nV*64=8uV.
After using the PGA, we can reduce the number of digits required for the ADC. Now we can use the ADS1146 with a 16-bit full-scale input of 5V to complete the design. At this time, the 1LSB of the ADS1146=5V/65536=76uV, from the amplified 1.6V 1.6V/76uV=20,000 data can be obtained in the differential mode signal! At the same time, ADS1146 also has a complete differential input stage, which can complete the work of the entire signal chain.
(2) Use ADS1147 to complete the measurement of 3-wire RTD resistance
ADS1147 (16 bits) and ADS1247 (24 bits) are △-∑ ADCs with built-in current sources. This type of ADC is designed for sensors such as RTDs that require current source excitation. Its fully differential input, PGA amplifier and ultra-high precision help We save a whole bunch of op amps and resistors.
In addition to pressure signals, slow-changing signals such as temperature signals require high-precision measurement, and some low-frequency AC small signals with a large dynamic range need to be collected with a slightly higher sampling rate and high precision, such as vibration signals and seismic signals. At this time The throughput rate of tens or hundreds of Hz is difficult to meet the requirements, and the ADS127x launched by TI, the industry's fastest Σ-Δ ADC with both DC and AC precision, is very suitable for this kind of demand.
As shown above, in terms of AC accuracy, compared with industrial Σ-Δ ADCs with similar DC accuracy, ADS127x has a wider frequency response and can sample analog input signals with a bandwidth of 62KHz; and in terms of DC accuracy, compared with the same sampling Compared with high-speed audio frequency ∑-△ ADC, ADS127x fully retains the DC accuracy of measurement.