AD5934 datasheet(4/6 Pages) AD

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CN-0217

Circuit Note

Rev. A | Page 4 of 6

Example 2: kΩ Impedance Range

Using an RCAL of 99.85 kΩ, a wide range of unknown impedances

were measured according to the setup conditions listed in Table 4.

Figure 6 to Figure 10 document accuracy results. To improve

the overall accuracy, select an RCAL value closer to the unknown

impedance. For example, in Figure 9, an RCAL closer to the ZC

value of 217.5 kΩ is required. If the unknown impedance range

is large, use more than one RCAL resistor.

Table 4. kΩ Impedance Range Setup for VDD = 3.3 V Supply

Voltage

Parameter

Value

Voltage Peak-to-Peak (V p-p)

0.198 V (Range 4)

Number of Settling Time Cycles

MCLK

16 MHz

RCAL

99.85 kΩ

RFB

100 kΩ

Excitation Frequency Range

30 kHz to 50 kHz

Unknown Impedances

R0 = 99.85 kΩ, R1 = 29.88 kΩ,

R2 = 14.95 kΩ, R3 = 8.21 kΩ,

R4 = 217.25 kΩ, C5 = 150 pF,

(ZC = 26.5 kΩ at 40 kHz),

C6 = 47 pF (ZC = 84.6 kΩ at

40 kHz)

100

110

120

FREQUENCY (kHz)

MEASURED

IDEAL

Figure 6. Magnitude Result for ZC = 47 pF, RCAL = 99.85 kΩ

–90.3

–90.2

–90.1

–90.0

–89.9

–89.8

–89.7

–89.6

–89.5

–89.4

–89.3

FREQUENCY (kHz)

Figure 7. Phase Result for ZC = 47 pF, RCAL = 99.85 kΩ

FREQUENCY (kHz)

8160

8180

8200

8220

8240

8260

8280

IDEAL

Figure 8. ZC = 8.21 kΩ, RCAL = 99.85 kΩ

FREQUENCY (kHz)

213.5

214.0

214.5

21.50

215.5

216.0

216.5

217.0

217.5

218.0

218.5

IDEAL

Figure 9. ZC = 217.25 kΩ, RCAL = 99.85 kΩ

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