The market demand for industrial applications is increasing day by day, and data acquisition systems are the key equipment among them. They are usually used to detect temperature, flow, liquid level, pressure and other physical quantities, and then convert the analog signals corresponding to these physical quantities into high-resolution digital information, which is then further processed by software.
Such systems are increasingly demanding precision and speed. These data acquisition systems are composed of amplifier circuits and analog-to-digital converters (ADC), and their performance has a decisive influence on the system.
However, the input driver of the ADC also affects the overall accuracy. The driver is used to buffer and amplify the input signal. In addition, a bias signal must be added or a fully differential signal must be generated to cover the ADC’s input voltage range and meet its common-mode voltage requirements. The original signal must not be changed in the process.
Programmable gain instrumentation amplifiers (PGIA) are commonly used as input drivers. In this article, we propose a combination of input driver and ADC, through this combination can achieve more accurate conversion results, thereby building a high-quality data acquisition system.
For example, LTC6373 is a PGIA suitable for high-precision data acquisition systems. In addition to the fully differential output, it also has high DC accuracy, low noise, low distortion (see Figure 2) and a high bandwidth of 4 MHz, with a gain of 1/4 to 16. ADC can be directly driven by it, so it is suitable for many signal conditioning applications.
The circuit in Figure 1 shows an example of using the LTC6373 to drive a precision ADC. The ADC is an AD4020 with a 20-bit resolution of 1.8 MSPS.
Figure 1. Example of a circuit driving a precision ADC.
In this circuit, the LTC6373 is DC-coupled at the input and output, so there is no need to use a transformer to drive the ADC. The gain can be set from 0.25 V/V to 16 V/V via pins A2/A1/A0. In Figure 1, the LTC6373 uses a differential input to differential output configuration and ±15 V symmetrical supply voltage. Alternatively, the input can also be a single-ended input, while the output is still a differential output.
In Figure 1, the output common-mode voltage passes through V OCM Pin is set to V REF /2. In this way, the output level conversion of LTC6373 can be realized. Each output of LTC6373 is 0 V to V REF Change between, so there is a 2× V at the ADC input REF The amplitude of the differential signal.
The RC network between the output of the LTC6373 and the input of the ADC forms a single-pole low-pass filter, which can reduce the current glitches generated when the capacitor is switched at the input of the ADC. At the same time, the low-pass filter limits broadband noise.
Figure 2 shows the signal-to-noise ratio (SNR) and total harmonic distortion (THD) of the LTC6373, which drives the AD4020 SAR ADC (high impedance mode) over the entire input voltage range (10 V pp). At a throughput of 1.8 MSPS, the filter resistance (R FILTER When) is 442 Ω, a satisfactory effect can be obtained. At 1 MSPS or 0.6 MSPS, the manufacturer recommends R FILTER It is 887 Ω.
Figure 2. Using LTC6373 to drive AD4020 SNR (left) and THD (right) performance.
The LTC6373 can drive most SAR ADCs with differential inputs, without the need for additional ADC drivers. However, in some applications, a separate ADC driver can be used between the LTC6373 and the precision ADC to further improve the linearity of the signal chain.
in conclusion
The circuit shown in Figure 1 is optimized for fast, high-precision data acquisition systems. Therefore, the outstanding characteristics of LTC6373 contribute to signal conditioning of the sensor output signal. With the help of the online tool ADI Precision Studio, especially the ADC driver tool contained therein, ADI can provide more support for the design of such amplifier stages, filters and linear circuits. For more information, please visit tools.analog.com/en/precisionstudio.