AD7745 datasheet(22/28 Pages) AD

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AD7745/AD7746

Rev. 0 | Page 22 of 28

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DATA

CAPDIFF = 1

± 4pF

CDC

EXC

CIN(+)

CIN(–)

15 ... 19pF

(17

± 2pF)

15 ... 19pF

(17

± 2pF)

CAPDAC(+)

17pF

CAPDAC(–)

17pF

Figure 34. Using CAPDAC in Differential Mode

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DATA

CAPDIFF = 1

± 4pF

CDC

EXC

CIN(+)

CIN(–)

13 ... 21pF

(17

± 4pF)

17pF

CAPDAC(+)

17pF

CAPDAC(–)

17pF

Figure 35. Using CAPDAC in Differential Mode

PARASITIC CAPACITANCE TO GROUND

DATA

CDC

EXC

CGND1

CIN

CGND2

Figure 36. Parasitic Capacitance to Ground

The CDC architecture used in the AD7745/AD7746 measures

the capacitance CX connected between the EXC pin and the

CIN pin. In theory, any capacitance CP to ground should not

affect the CDC result (see Figure 36).

The practical implementation of the circuitry in the chip

implies certain limits and the result is gradually affected by

capacitance to ground. See the allowed capacitance to GND in

the specification table for CIN and excitation. Also see the

typical performance characteristics shown in Figure 9, Figure

10, and Figure 11.

PARASITIC RESISTANCE TO GROUND

DATA

CDC

EXC

RGND1

CIN

RGND2

Figure 37. Parasitic Resistance to Ground

The AD7745/AD7746 CDC result would be affected by a leak-

age current from the CX to ground, therefore the CX should be

isolated from the ground. The influence of the leakage current

varies with the power supply voltage. The following limits can

be used as a guideline for the allowed leakage current or the

equivalent resistance between the CX and ground (Figure 37).

VDD ≈ 5 V: IGND < 150 nA (that is, RGND > 30 MΩ)

VDD ≥ 3 V: IGND < 60 nA (that is, RGND > 50 MΩ)

VDD ≥ 2.7 V: IGND < 30 nA (that is, RGND > 100 MΩ)

A higher leakage current to ground results in a gain error, an

offset error, and a nonlinearity error. See the typical

performance characteristics shown in Figure 12 and Figure 13.

PARASITIC PARALLEL RESISTANCE

DATA

CDC

EXC

CIN

Figure 38. Parasitic Parallel Resistance

The AD7745/AD7746 CDC measures the charge transfer

between EXC pin and CIN pin. Any resistance connected in

parallel to the measured capacitance CX (see Figure 38), such as

the parasitic resistance of the sensor, also transfers charge.

Therefore, the parallel resistor is seen as an additional

capacitance in the output data. The equivalent parallel

capacitance (or error caused by the parallel resistance) can be

approximately calculated as

EXC

Where RP is the parallel resistance and CEXC is the excitation

frequency. See the typical performance characteristics shown in

Figure 14.

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