DAC7750: DAC7750 – Overrange / Underrange Handling for pH Scaling (0–20 mA / 4–20 mA mode)

I am working with the DAC7750 current output DAC and implementing pH-to-current conversion for an industrial transmitter application. 

Requirement 

We have a pH range of 0 to 14, which maps to: 

• 0–20 mA mode → 0 pH = 0 mA, 14 pH = 20 mA 

• 4–20 mA mode → 0 pH = 4 mA, 14 pH = 20 mA 

Additionally, we have a requirement to support: 

• Overrange (e.g., pH > 14 → current slightly above 20 mA, say up to ~20.5 mA) 

• Underrange (e.g., pH < 0 → current slightly below 4 mA in 4–20 mode) 

Current Understanding 

From the datasheet, DAC7750 supports: 

• 0–20 mA 

• 4–20 mA 

• 0–24 mA output ranges 

However, it appears that: 

• In 4–20 mA mode, output is limited to 4–20 mA (no true overrange possible) 

• In 0–20 mA mode, output is limited to 0–20 mA 

• The DAC saturates at configured full-scale and does not internally support overrange/underrange signaling 

Proposed Approach 

To handle overrange/underrange, we are considering: 

• Initializing DAC in 0–24 mA mode 

• Using application-level scaling:  

• 0–20 mA → normal operating range 

20 mA → overrange region 

• <4 mA (for 4–20 behavior) → underrange region 

Example: 

• 0 pH → 0 mA (or 4 mA for 4–20 behavior) 

• 14 pH → 20 mA 

• pH > 14 → allow current up to ~24 mA (within DAC capability) 

Questions 

1. Is it recommended to use 0–24 mA mode to implement overrange/underrange behavior for 0–20 mA and 4–20 mA applications? 

2. Are there any limitations or accuracy concerns when operating in 0–24 mA mode but using only a subset (0–20 mA/4-20mA) for normal operation? 

3. Does DAC7750 provide any internal mechanism for overrange indication, or is this strictly application-level handling? 

4. Are there any calibration or gain considerations when using extended range like this? 

5.  What are the configurations and hardware confirmations required for Accuracy and Linearity purpose for 600ohm load condition
6. What is the importance of Boost circuitry and without boost circuitry, is there any accuracy effect on this?  

Goal 

We want to achieve behavior similar to industrial transmitters: 

• Normal: 4–20 mA 

• Overrange: ~20-20.5 mA 

• Underrange: ~3.5–4 mA 

 DAC Schematic FYR,
 

Any guidance or confirmation on the recommended approach would be very helpful. 

Awaiting for your response

Thanks in advance!