AD5933 datasheet(5/6 Pages) AD

5 / 6 page

Circuit Note

CN-0217

Rev. A | Page 5 of 6

100

120

FREQUENCY (kHz)

Figure 10. Magnitude Results for Example 2: R1, R2, R3, C5, C6

Example 3: Parallel R-C (R||C) Measurement

An R||C type measurement was also made using the configuration,

using an RCAL of 1 kΩ, an R of 10 kΩ, and a C of 10 nF, measured

across a frequency range of 4 kHz to 100 kHz. The magnitude

and phase results vs. ideal are plotted in Figure 11 and Figure 12.

Table 5. R||C Impedance Range Setup for VDD = 3.3 V Supply

Voltage

Parameter

Value

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

0.383 V (Range 3)

Number of Settling Time Cycles

MCLK

16 MHz

RCAL

1 kΩ

RFB

1 kΩ

Excitation Frequency Range

4 kHz to 100 kHz

Unknown Impedance R||C

R = 10 kΩ, C = 10 nF

500

1000

1500

2000

2500

3000

3500

4000

104

FREQUENCY (kHz)

IDEAL

MEASURED

Figure 11. Magnitude Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ

104

FREQUENCY (kHz)

IDEAL

MEASURED

–95

–90

–85

–80

–75

–70

–65

–60

Figure 12. Phase Results for ZC = 10 kΩ||10 nF, RCAL = 1 kΩ

Setup and Test

The evaluation board software is the software used on the

EVAL-AD5933EBZ. Refer to the technical note available on the

CD provided with the evaluation board for details on the board

setup. Note that there are alterations to the schematic. Link

connections on the EVAL-AD5933EBZ board are listed in Table 4.

In addition, note that the location for RFB is located at R3 on the

evaluation board, and the location for ZUNKNOWN is C4.

Table 6. Link Connections for EVAL-AD5933EBZ

Link Number

Default Position

LK1

Open

LK2

Open

LK3

Insert

LK4

Open

LK5

Insert

LK6

Complete setup and operation for the hardware and software

for the evaluation board can be found in User Guide UG-364.

COMMON VARIATIONS

Other op amps can be used in the circuit, such as the ADA4528-1,

AD8628, AD8629, AD8605, and AD8608.

Switching Options for System Applications

For this particular circuit, the ZUNKNOWN and RCAL were interchanged

manually. However, in production, use a low on-resistance switch.

The choice of the switch depends on how large the unknown

impedance range is and how accurate the measurement result

needs to be. The examples in this circuit note use just one

calibration resistor, and so a low on-resistance switch, such as

the ADG849, can be used as shown in Figure 13. Multichannel

switch solutions, such as the quad ADG812, can also be used.

The errors caused by the switch resistance on the ZUNKNOWN are

removed during calibration, but by choosing a very low RON

switch, the effects can be further minimized.

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