Can a low noise PGA be implemented using a digital potentiometer?

littleshrimp

Can a low noise PGA be implemented using a digital potentiometer? [Copy link]

As shown in the figure below, I want to add a PGA (Programmable Gain Amplifier) to the front end of a high-bit ADC. I see that the noise of the off-the-shelf PGA is usually quite high. I don't know much about the module circuit, so I would like to ask: 1. If there is an ultra-low-noise amplifier, can a digital potentiometer be used to achieve an ultra-low-noise PGA? If the gain and temperature drift are not considered, which parameters of the digital potentiometer will affect which indicators of the PGA? 2. If a PGA is used, can the ADC driver (the PGA uses the same amplifier as the ADC driver) before the ADC be omitted?

dontium

The idea of the original poster's diagram is very good. Now let's calculate the actual situation. A 24-bit ADC uses the commonly used voltage of 3.3V, or full scale. That is, it can distinguish 3.3V/16777215 =2 * 10^-7 V, which is 0.2uV. That is, the error of the previous amplifier (offset, drift, etc.) must be less than 0.2uV to be meaningful. This is the error at the output of the amplifier (the indicators of the amplifier are generally converted to the input end). If the gain of the amplifier is 20 times, then its error must be less than 0.2uV/20 = 0.01uV. Look at this quantity, this is an extremely weak signal. The original poster's diagram still amplifies the positive and negative separately, which adds errors to the amplifier. In fact, adding a follower in front of the ADC also has very high requirements. In general applications, a way is found to reduce the impedance of the signal source, so there is no need to add a buffer stage in front of the ADC. This is beneficial to improving accuracy

littleshrimp

dontium posted on 2015-12-1 13:42 The idea of the picture by the original poster is very good. Now let's calculate the actual situation. 24-bit ADC, according to the commonly used voltage 3.3V, or full scale. That is, it...

The first picture is a reference circuit of a 24-bit SAR ADC LT2380-24 from Lingte. The ADC driver uses the LT6362 output differential operational amplifier/SAR ADC driver. The second picture is some parameters of LT6362. The maximum input offset voltage is 600uV, and the offset voltage drift is also 0.9uV per degree Celsius. Is it difficult for the current ADC driver to support 24-bit resolution ADC? If it must be used, the corresponding resolution must be lost? If we want to achieve higher measurement accuracy, can we solve it through additional calibration? Will the offset voltage drift calibration be very troublesome? I have also checked some high-bit ADC drivers of ADI, and the parameters are basically the same as LT6362. Assuming that the use of ADC driver is inevitable, if we want to measure small signals on this basis, we need to do some amplification at the front end. We will use high-precision and low-temperature drift fixed resistors, right? Now the question is, if we use digital potentiometers to replace fixed resistors to implement programmable amplifiers, excluding the shortcomings of digital potentiometers in terms of accuracy and temperature drift, will the other parameters of the digital potentiometers have a very large impact on this circuit?

dontium

The number of bits of ADC can only indicate its resolution, while the accuracy, different chips have different standards, that is, the actual accuracy will not be 24 bits. Therefore, when choosing the pre-driver, it should also be determined according to the specific situation. Some indicators of the amplifier can be adjusted to reach the best, and it is possible that some can be much higher than the manual indicators. ADC can also be calibrated, but some indicators can be calibrated, while some cannot. The drift and noise of the resistor are generally small compared to the amplifier, but its influence should also be paid attention to in some occasions. The digital potentiometer is easy to use, but its performance is definitely not as good as the potentiometer made of resistors.

littleshrimp

dontium posted on 2015-12-1 15:44 The number of bits of ADC can only indicate its resolution, while the accuracy has different standards for different chips, that is, the actual accuracy will not be 24 bits. So...

Seeing that the resistance noise of the digital potentiometer is not small, it seems that the performance of the DIY PGA will not be very good.