High internal resistance design reference, don’t understand?

bigbat

High internal resistance design reference, don’t understand? [Copy link]

This is the design reference of ADI's pH meter. The parameters say that it can measure 1GΩ internal resistance signal. The reference standard is 100MΩ.

First one:

This is the block diagram, there is no doubt about it.

Second picture:

The AD8603 in the figure should be a follower. But there are two questions.

1. In the figure, the negative electrode of the sensor is connected to a 5kΩ resistor, not directly to the ground?

2. In the figure, the positive electrode of the sensor is connected to a 1MΩ resistor, but it is not directly connected to the input terminal of the op amp?

littleshrimp

The 5k resistor should provide a bias level for the reference voltage. 5k*210uA=1.05V. The 1M resistor in series with the AD8603 may be used to limit the current and prevent the external high voltage from damaging the amplifier.

guitianxia

First of all, I would like to give a thumbs up to the brother above, an expert...

The high internal resistance AD8603 has unclear design

Now our company has launched a new product AD8605

Product Features

Low offset voltage: 65 μV (max)

Low input bias current: 1 pA (max)

Low noise: 8 nV/√Hz

Wide bandwidth: 10 MHz

High open loop gain: 1000 V/mV

Unity-gain stable

Single power supply: 2.7 V to 5.5 V

5-lead WLCSP (single AD8605 amplifier) and 8-lead WLCSP (dual AD8606 amplifier)

bigbat

From the figure, we can see that the sensor electrode is not connected to the ground, but to 1/3 of the RFIN voltage. So why is it not connected to the ground? Is it because the electrode has a negative output or because the op amp is a single power supply?

bigbat

guitianxia posted on 2019-10-31 22:51 First of all, I would like to give a thumbs up to the brother above, an expert... This high internal resistance AD8603 has unclear design...

AD8603 is an op amp, and in this reference it is a follower, what is unclear? Your new device is in WLCSP package, and the welding cost here is high, so we will not use it!

maychang

What littleshrimp said on the second floor is completely correct. The 5k resistor provides a bias for the reference voltage, and the 1M resistor has no effect on the input signal (because the input current of AD8603 is extremely small, and the 1M resistor has no voltage drop), it only plays a protective role.

bigbat

Original link of the article

“A typical pH probe electrode is made of glass, which forms an extremely high resistance, ranging from 1 MΩ to 1 GΩ, acting as a resistor in series with the pH voltage source , as shown in Figure 2.

The buffer amplifier bias current flowing through this series resistor will introduce an offset error into the system . To isolate the circuit from this high source resistance, a high input impedance, ultralow input bias current buffer amplifier is required in this application. The AD8603 is used as a buffer amplifier for this application, as shown in Figure 2. The low input current of the AD8603 minimizes the voltage error generated by the bias current flowing through the electrode resistance.

For a pH probe with a 1 GΩ series resistance at 25°C, the offset error is 0.2 mV (0.0037 pH) for a typical input bias current of 200 fA. Even at the maximum input bias current of 1 pA, the error is only 1 mV.

The cutoff frequency of the 10 kΩ/1 μF low-pass noise filter for the buffer amplifier output is f = 1/2πRC, or 16 Hz.

Guarding, shielding, high insulation resistance standoffs, and other such standard picoamp techniques must be utilized to minimize leakage at the high impedance inputs of the AD8603 buffer.”

The red part says that the internal resistance of the pH electrode is very large, but wouldn't it be even larger if a resistor is connected in series? What's the benefit? If it is to protect the amplifier, then it is not necessary. The op amp itself is protected, and the internal resistance of the op amp is very high. High voltage will only saturate the amplifier.

bigbat

littleshrimp posted on 2019-10-31 21:31 The 5k resistor should provide a bias level for the reference voltage. 5k*210uA=1.05V. The 1M resistor in series with AD8603 may be to limit the current to prevent external high voltage loss...

Looking at the first block diagram, there is only a 10KΩ/1uF filter, without the series 1M resistor.

maychang

bigbat posted on 2019-11-1 10:21 Original link of the article "Typical pH probe electrodes are made of glass, which can form extremely high resistances ranging from 1 MΩ to 1 GΩ ...

The internal resistance of the pH electrode is much larger than that of a 1M ohm resistor. Connecting a 1M ohm resistor in series has little effect on the measurement results. "Wouldn't it be larger if a resistor is connected in series?" It is indeed larger, but it has almost no effect on the measurement results.

"If it is to protect the amplifier, then it is not necessary." Because the pH electrode is outside the instrument, it is still necessary to connect a series resistor to protect the op amp. The op amp itself has no protection, and applying a high voltage externally will damage the op amp.

maychang

bigbat posted on 2019-11-1 10:21 Original link of the article "Typical pH probe electrodes are made of glass, which can form extremely high resistances ranging from 1 MΩ to 1 GΩ ...

For AD8603, if the input signal exceeds the supply voltage range, it may be damaged. Because the electrodes are external, it is inevitable that human hands or tools touch the electrode leads and cause damage to the op amp.

littleshrimp

bigbat published on 2019-11-1 10:21 Original link of the article "Typical pH probe electrodes are made of glass, which can form extremely high resistances ranging from 1 MΩ to 1 GΩ ...

Page 12 of the AD8603 Chinese data sheet C version introduces the design of input protection "Input overvoltage protection VIN VOUT TIME (4s/DIV) Figure 41. No phase reversal If a voltage higher than 1 V above the power supply is applied to either input, a current limiting series resistor is recommended. If both inputs are used, a series resistor should be added to each input for protection. To ensure effective protection, the current should be limited to less than 5 mA. The value of the current limiting resistor can be determined by the following formula: (VIN VS)/(RS + 200 Ω) ≤ 5 mA "