We had an enquiry from a customer whose 4-20mA loop ran a long way from the field instrument back to the control room, then on to a chart recorder wired in series with the PLC input. By the time they added up the wiring resistance and the recorder’s own input resistance, the total load on the loop was pushing past what their existing signal conditioner could drive — and the output was clipping instead of tracking the signal properly.
Every current-output instrument has a maximum load resistance it can drive before the output voltage needed exceeds what its internal power supply can produce. Add up enough cable resistance, plus two or three devices wired in series on the same loop, and a standard conditioner rated for around 750Ω can run out of headroom. The signal doesn’t fail outright — it just can’t reach the top of its range, so high readings get clipped or the loop drops out intermittently.
For this, we would normally recommend the MTT MS3704HI, a slim plug-in high-level signal conditioner built as a high-output-load model, able to drive up to 1kΩ of total loop resistance instead of the standard 750Ω. It accepts a DC current or voltage input and delivers a standard 4-20mA DC output, isolated from both the input and the power supply. Front-accessible zero and span trimmers make on-site adjustment quick, and it accepts AC or DC power, so it drops into most existing panel designs without extra conversion hardware.
Before we quote, we would normally check:
- Total loop resistance — cable run plus every device wired in series on the loop
- Input signal type and range
- Power supply available at the panel (AC or DC)
Hope this helps — send us your loop resistance calculation and input signal, and we can confirm the right model.
| Input Types | DC current (100μA to 200mA span) or DC voltage (-300V to 300V); standard 4-20mA DC with 250Ω input resistance |
| Allowable Input | 40mA DC max. continuous (current); 30V DC max. continuous (voltage) |
| Output | 4-20mA DC, 16mA span |
| Maximum Output Load | 1kΩ |
| Zero & Span Adjustment | ±5% of span via front-accessible trimmers |
| Isolation | 4-way isolation between input, output, power, and ground |
| Power Supply | 100-240V AC (47-63Hz, 6.0VA max), 24V DC ±10% (1.5W max), or 100-240V DC (2.5W max) |
| Accuracy | Better than ±0.1% of span (at 25°C±5°C) |
| Temperature Effect | ±0.2% of span per 10°C change |
| Response Time | 85ms maximum (0-90%) |
| CMRR | Minimum 100dB at 500V AC |
| Insulation Resistance | 100MΩ minimum at 500V DC |
| Dimensions (W x H x D) | 29mm x 86mm x 125mm |
| Weight | Main unit up to 120g; socket up to 80g |
| Mounting | Wall or DIN rail mounting, plug-in socket base |
| Terminals | M3.5 screw terminals, 0.8-1.0Nm torque |
| Operating Temperature | -5 to 55°C ambient (storage: -10 to 60°C) |
| Operating Humidity | 5-90% RH, non-condensing |
Frequently Asked Questions
What does the “HI” in MS3704HI mean?
It stands for high output load — the MS3704HI can drive up to 1kΩ of total loop resistance, compared to around 750Ω for the standard MS3704.
What is the MS3704HI used for?
It’s used where the standard signal conditioner’s output can’t drive the full loop — typically a long cable run, or several devices (recorders, indicators, PLC inputs) wired in series on the same 4-20mA loop.
What’s the difference between the MS3704HI and the standard MS3704?
Both condition DC current or voltage signals into a 4-20mA output, but the MS3704HI is built to drive a higher load resistance (1kΩ vs 750Ω). The MS3704HI is single output only, while the standard MS3704 also offers an isolated dual output option.
Does the MS3704HI offer a dual output option?
No — it’s a single 4-20mA output model. If you need to feed two separate systems from one signal, the standard MS3704 dual-output version would be the better fit.
How do I know which MS3704 model I need?
Add up your loop’s total resistance (cable plus every series-wired device), confirm your input signal, and let us know if you need a single or dual output. We’ll confirm the right model before you order.
Download MTT MS3704HI Slim Plug-In High-Level Signal Conditioner information here
Spec Sheet: MTT MS3704HI Spec Sheet
Manual: MTT MS3704HI Manual
