Source: https://electronics.stackexchange.com/questions/171417/purpose-of-input-resistor-in-adc-circuit

What is the implication of the 1 Megaohm resistor in the schematics attached? Will I be able to detect the condition of the motor (running, not running, stalled)?

The 1M resistor would allow high voltage spikes to exist at the motor without damaging the ADC input. By limiting the current into the ADC input protection network, even a 1kV spike would be rendered harmless (unless the resistor arced over).
1M 電阻器允許馬達上存在高電壓尖峰，而不會損壞 ADC 輸入。透過限制進入 ADC 輸入保護網路的電流，即使是 1kV 尖峰也將變得無害（除非電阻器發生電弧放電）。

However most ADC inputs are not very tolerant of such a high input resistance unless they have a built in buffer amplifier- often a few K ohms is the maximum recommended. Even if there is a buffer (rare) the datasheet leakage spec (you say 1uA) is usually too high to guarantee reasonable accuracy (typical leakage might be pA at room temperature if buffered). So, the design seems somewhat dubious at first blush, though it may indeed function. It's doubtful actual spikes exceeding a few tens of volts would occur at the power to an H-bridge, so a resistor value of a few K might be fine.
然而，大多數 ADC 輸入不能承受如此高的輸入電阻，除非它們具有內建緩衝放大器 - 通常建議的最大值為數千歐姆。即使有緩衝器（很少見），數據表洩漏規格（您說 1uA）通常也太高，無法保證合理的精度（如果有緩衝，在室溫下典型洩漏可能為 pA）。因此，乍一看這個設計似乎有點可疑，儘管它確實可以發揮作用。 H 橋電源上是否會出現超過幾十伏特的實際尖峰值得懷疑，因此幾 K 的電阻值可能沒問題。

The diode is presumably to prevent the current sense resistor from limiting the motor stall/start current. It will kick in at around 270mA motor current. If the motor is 'plugged' it's quite possible to get around double the stall current momentarily.
此二極體可能是為了防止電流檢測電阻器限制馬達堵轉/啟動電流。它將在馬達電流約為 270mA 時啟動。如果馬達被“插入”，則很可能會暫時繞過雙倍的失速電流。

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