Datasheet MAX6627 (Maxim) - 8

link to page 8 link to page 8 link to page 4 link to page 4 MAX6627/MAX6628 Remote ±1°C Accurate Digital Temperature Sensors with SPI-Compatible Serial Interface Filter high-frequency electromagnetic interference (EMI) widths and spacings recommended in Figure 3 are at DXP and DXN with an external 2200pF capacitor not absolutely necessary (as they offer only a minor connected between the two inputs. This capacitor can improvement in leakage and noise), but use them be increased to about 3300pF (max), including cable where practical. capacitance. A capacitance higher than 3300pF 8) Placing an electrically clean copper ground plane introduces errors due to the rise time of the switched- between the DXP/DXN traces and traces carrying current source. high-frequency noise signals helps reduce EMI.
PCB Layout Twisted Pair and Shielded Cables
1) Place the MAX6627/MAX6628 as close as practical For remote-sensor distances longer than 8in, or in particularly to the remote diode. In a noisy environment, such as noisy environments, a twisted pair is recommended. Its a computer motherboard, this distance can be 4in to practical length is 6ft to 12ft (typ) before noise becomes 8in, or more, as long as the worst noise sources (such a problem, as tested in a noisy electronics laboratory. For as CRTs, clock generators, memory buses, and ISA/ longer distances, the best solution is a shielded twisted PCI buses) are avoided. pair like that used for audio microphones. For example, 2) Do not route the DXP/DXN lines next to the deflection Belden #8451 works well for distances up to 100ft in a coils of a CRT. Also, do not route the traces across a noisy environment. Connect the twisted pair to DXP and fast memory bus, which can easily introduce +30°C DXN and the shield to ground, and leave the shield’s error, even with good filtering. Otherwise, most noise remote end unterminated. Excess capacitance at DXN or sources are fairly benign. DXP limits practical remote-sensor distances (see Typical 3) Route the DXP and DXN traces parallel and close Operating Characteristics). to each other, away from any high-voltage traces For very long cable runs, the cable’s parasitic capacitance such as +12VDC. Avoid leakage currents from PCB often provides noise filtering, so the recommended 2200pF contamination. A 20MΩ leakage path from DXP to capacitor can often be removed or reduced in value. Cable ground causes approximately +1°C error. resistance also affects remote-sensor accuracy. A 1Ω series 4) Connect guard traces to GND on either side of the resistance introduces about +1/2°C error. DXP/DXN traces (Figure 3). With guard traces in place, routing near high-voltage traces is no longer an issue. 5) Route as few vias and crossunders as possible to GND minimize copper/solder thermocouple effects. 10mils 6) When introducing a thermocouple, make sure that 10mils DXP both the DXP and the DXN paths have matching MINIMUM thermocouples. In general, PCB-induced 10mils DXN thermocouples are not a serious problem. A copper 10mils solder thermocouple exhibits 3µV/°C, and it takes GND approximately 200µV of voltage error at DXP/DXN to cause a +1°C measurement error, so most parasitic Figure 3. Recommended DXP/DXN PC Traces thermocouple errors are swamped out. 7) Use wide traces. Narrow traces are more induc- tive and tend to pick up radiated noise. The 10mil www.maximintegrated.com Maxim Integrated │

8