Circuit Note

CN-0359

analog.com

Rev. C | 8 of 10

lead resistance error. The RTD resistance can be calculated using

Equation 16:

Rx=ADC8−ADC5

ADC7−ADC8

×1.5 kΩ

(16)

where:

Rx is the RTD resistance value.

ADC5 is the AIN5 input voltage in V.

ADC7 is the AIN7 input voltage in V.

ADC8 is the AIN8 input voltage in V.

Figure 9 shows the 2-wire RTD configuration where there is no

compensation for lead resistance. The RTD resistance for 2-wire

configuration can be calculated as Equation 17:

Rx=ADC8−ADC9

ADC7−ADC8

×1.5 kΩ ‐ 2 RP

(17)

where:

Rx is the RTD resistance value.

ADC7 is the AIN7 input voltage in V.

ADC8 is the AIN8 input voltage in V.

ADC9 is the AIN9 input voltage in V.

RP is the parasitic resistance value.

Figure 9. Configuration for 2-Wire RTD Connection

The 2-wire configuration is the lowest cost circuit and is suitable

for less critical applications, short RTD connections, and higher

resistance RTDs such as Pt1000.

POWER SUPPLY CIRCUITRY

To simplify system requirements, all the required voltages (±15 V

and +3.3 V) are generated from a single 4 V to 7 V supply, as

shown in Figure 10.

The ADP2300 buck regulator generates the 3.3 V supply for the

board. The design is based on the downloadable ADP230x Buck

Regulator Design Tool.

The ADP5072 switching regulator generates independently regulat-

ed +15 V and -15 V supply rails for the different amplifiers. This

design follows the typical application circuit shown in the ADP5072

data sheet, and uses the recommended component values for a

+3.3 V to ±15 V application.

Figure 10. Power Supply Circuitry

Figure 11 shows the LCD backlight driver circuit. The AD8592 op

amp acts as a 30 mA current source to supply the LCD backlight

current. The AD8592 can source and sink up to 250 mA, and the

100 nF capacitor ensures a soft startup.

Figure 11. LCD Backlight Drivers

COMMON VARIATIONS

The system shown in the CN0359 uses the ADuCM360 precision

analog microcontroller for a highly integrated conductivity measure-

ment. If the user desires a discrete ADC, the AD7794 24-bit, Σ-Δ

ADC is a good choice.

If board space is at a premium, the ADP2300 buck regulator can

be replaced with a μModule

® part such as the LTM8074. While a

more expensive solution compared to the ADP2300, the LTM8074

is a highly integrated device with the inductor already built into its

package and does not require a catch diode. It is also designed

with the low noise Silent Switcher® architecture, which allows for

minimal electromagnetic interference (EMI) during operation.

Certain conductivity measurement applications may have additional

requirements such as isolation or a small hardware footprint. As