The matching problem between op amp and ADC

呜呼哀哉

The matching problem between op amp and ADC [Copy link]

As shown in the figure

The op amp with positive and negative 15V power supply and the ADC with 5V power supply after stage are unbuffered according to the input range given by AD7790.

GND-30mV to Vdd+30mV, basically no clamping diode can achieve a voltage drop of tens of millivolts, so I thought of adding a resistor before the clamping diode so that the current after the maximum output of the op amp minus the clamping value does not exceed the output capacity of the op amp. Add a resistor later, and the voltage entering the ADC is the clamping diode voltage drop minus the resistor voltage drop. Is this self-evident method feasible? What is the correct method?

In addition, AD7790 says that the previous RC filtering parameters are the following combination to ensure no distortion.

The problem is that the average input current of ±400nA/V is not small, which is equivalent to the ADC equivalent input impedance of 2.5M. When combined with the 22.8K/50pF, the voltage division error is already within 1%. Do you understand correctly?

Is the filtering method for this pseudo-differential input as shown in the pencil drawing? If so, should C be twice the C of the given combination? Or should it be directly RC to ground?

chunyang

The op amp output circuit should not be connected to resistors, otherwise it is equivalent to increasing the internal resistance of the signal source, which will reduce the accuracy. If the resistance value is insufficient, there is no effective protection. The larger the value, the more obvious the impact on the accuracy. The correct response can be to connect a single-power rail-to-rail op amp with a 5V power supply to the original op amp output, followed by a follower or proportional amplifier, integrator, etc. The specific needs to be considered as a whole to obtain the best benefits. The op amp input is then clamped for protection, and the front-end design should pay attention to minimizing the probability of overvoltage.

Fred_1977

I don't understand why the clamping diode is needed? Does the ADC reference design require this?

呜呼哀哉

Fred_1977 posted on 2022-1-20 21:44 I don't understand why the clamping diode is needed? Does the ADC reference design have this requirement?

The instrument amplifier used before has basically no rail-to-rail output, so the only way is to increase the instrument amplifier supply voltage to reach the ADC's full 5V input dynamic range.

呜呼哀哉

chunyang posted on 2022-1-20 18:21 The output circuit of the op amp should not be connected to a resistor, otherwise it will increase the internal resistance of the signal source and reduce the accuracy. If the resistance value is insufficient, there will be no effective protection. The larger the value, the more...

The front is the instrument release, the OP written casually, the INA track-to-track has never been used.

How do we explain the RC parameters of the 16-bit precision without distortion recommended by the AD7790 ADC? The maximum resistance is 22.8K, so adding a 510R resistor in front should not be a big problem.

As long as you understand the previous RC value theory, you should know whether this current limiting resistor is effective.

Does anyone know the theoretical calculation of RC?