Datasheet CN-0217 Datasheet (ANALOG DEVICES)

Circuit Note

Devices Connected/Referenced

Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0217.

AD5933

AD5934

1 MSPS, 12-Bit Impedance Converter,
Network Analyzer

250kSPS, 12-Bit Impedance Converter,
Network Analyzer

AD8606 Precision Low Noise Dual CMOS Op Amp

High Accuracy Impedance Measurements Using 12-Bit Impedance Converters

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0217 Circuit Evaluation Board

(EVAL-CN0217-EB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials

SCL

SDA

MCLK

OSCILLATOR

I2C

INTERFACE

REAL

REGISTER

IMAGINARY

REGISTER

1024-POINT DFT

AGND DGND

V

DD

ADC

(12 BITS)

V

DD

DVDDAVDD

DDS

CORE

(27 BITS)

TEMPERATURE

SENSOR

AD5933/AD5934

LPF

DAC

OUTPUT AMPLIFIER

GAIN

CIRCUIT FUNCTION AND BENEFITS

The AD5933 and AD5934 are high precision impedance
converter system solutions that combine an on-chip
programmable frequency generator with a 12-bit, 1 MSPS
(AD5933) or 250 kSPS (AD5934) analog-to-digital converter
(ADC). The tunable frequency generator allows an external
complex impedance to be excited with a known frequency.

The circuit shown in Figure 1 yields accurate impedance
measurements extending from the low ohm range to several
hundred kΩ and also optimizes the overall accuracy of the

AD5933/AD5934.

1.98V p-p

TRANSMIT S IDE

I-V

R

OUT

VDD/2

1.48V

VOUT

RFB

VIN

47nF

20kΩ

20kΩ

V

DD

50kΩ

50kΩ

A1, A2 ARE

½

AD8606

R

FB

A2

VDD/2

V

DD

−

A1

+

−

V

+

DD

50kΩ

50kΩ

CN-0217

1.98V p-p

Z

UNKNOWN

Figure 1. Optimized Signal Chain for Impedance Measurement Accuracy (Simplified Schematic, All Connections and Decoupling Not Shown)

Rev.0

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09915-001

FREQUENCY (Hz)

100

90

0

1k

100M10k 100k 1M 10M

80

70

20

60

50

30

10

40

A

V

= 100

AV = 10

AV = 1

VS = 2.7V

09915-002

OUTPUT IMPEDANCE (Ω)

CN-0217 Circuit Note

CIRCUIT DESCRIPTION

The AD5933 and AD5934 have four programmable output
voltage ranges; each range has an output impedance associated
with it. For example, the output impedance for a 1.98 V p-p
output voltage is typically 200 Ω (see Table 1).

Table 1. Output Series Resistance, R
for VDD = 3.3 V Supply Voltage,

Output Excitation

Range

Amplitude

Range 1 1.98 V p-p 200 Ω typ
Range 2 0.97 V p-p 2.4 kΩ typ
Range 3 0.383 V p-p 1.0 kΩ typ
Range 4 0.198 V p-p 600 Ω typ

This output impedance impacts the impedance measurement
accuracy, particularly in the low kΩ range, and should be taken
into account when calculating the gain factor. Please refer to the

AD5933 or AD5934 data sheets for more details on gain factor

calculation.

A simple buffer in the signal chain prevents the output
impedance from affecting the unknown impedance
measurement. A low output impedance amplifier should be
selected with sufficient bandwidth to accommodate the

AD5933/AD5934 excitation frequency. An example of the low

output impedance achievable is shown in Figure 2 for the

AD8605/AD8606/AD8608 family of CMOS op amps. The

output impedance for this amplifier for an A
1 Ω up to 100 kHz, which is the maximum operating range of the

AD5933/AD5934.

Figure 2. Output Impedance of AD8605/AD8606/AD8608

, vs. Excitation Range

OUT

Output Resistance,
R

OUT

of 1 is less than

V

Rev. 0 | Page 2 of 6

Matching the DC Bias of Transmit Stage to Receive Stage

The four programmable output voltage ranges in the AD5933/

AD5934 have four associated bias voltages (Table 2). For

example, the 1.98 V p-p excitation voltage has a bias of 1.48 V.
However, the current-to-voltage (I-V) receive stage of the

AD5933/AD5934 is set to a fixed bias of VDD/2 as shown in

Figure 1. Thus, for a 3.3 V supply, the transmit bias voltage is

1.48 V, while the receive bias voltage is 3.3 V/2 = 1.65 V. This
potential difference polarizes the impedance under test and can
cause inaccuracies in the impedance measurement.

One solution is to add a simple high-pass filter with a corner
frequency in the low Hz range. Removing the dc bias from the
transmit stage and re-biasing the ac signal to VDD/2 keeps the
dc level constant throughout the signal chain.

Table 2. Output Levels and Respective DC Bias for VDD =

3.3 V Supply Voltage

Output Excitation

Range

Amplitude Output DC Bias Level

1 1.98 V p-p 1.48 V
2 0.97 V p-p 0.76 V
3 0.383V p-p 0.31 V
4 0.198 V p-p 0.173 V

Selecting an Optimized I-V Buffer for the Receive Stage

The current-to-voltage (I-V) amplifier stage of the

AD5933/AD5934 can also add minor inaccuracies to the signal

chain. The I-V conversion stage is sensitive to the amplifier's
bias current, offset voltage, and CMRR. By selecting the proper
external discrete amplifier to perform the I-V conversion, the
user can choose an amplifier with lower bias current and offset
voltage specifications along with excellent CMRR, making the
I-V conversion more accurate. The internal amplifier can then
be configured as a simple inverting gain stage.

Selection of resistor R

still depends on the gain through the

FB

system as described in the AD5933/AD5934 data sheet.

Optimized Signal Chain for High Accuracy Impedance Measurements

Figure 1 shows a proposed configuration for measuring low
impedance sensors. The ac signal is high-pass filtered and re­biased before buffering with a very low output impedance
amplifier. The I-V conversion is completed externally before the
signal returns to the AD5933/AD5934 receive stage. Key
specifications that determine the required buffer are very low
output impedance, single-supply capability, low bias current,
low offset voltage, and excellent CMRR performance. Some
suggested parts are the AD4528-1, AD8628/AD8629, AD8605,
and AD8606. Depending on board layout, use a single-channel
or dual-channel amplifier. Use precision 0.1% resistors for both
the biasing resistors (50 kΩ) and gain resistors (20 kΩ and R
to reduce inaccuracies.

)

FB

Circuit Note CN-0217

35

30

25

20

15

10

5

0

29.95 30.00 30.05 30.10 30.15 30.20

10.3Ω

30Ω

1µF

30.25

FREQUENCY ( kHz )

MAGNITUDE (Ω)

09915-003

20

0

–20

–40

–60

–80

–100

29.95 30.00 30.05 30.10 30.15 30.20

1µF

30.25

FREQUENCY ( kHz )

PHASE (Degrees)

09915-004

10.3Ω, 30Ω

10.22

10.20

10.18

10.16

10.14

10.12

10.10

10.08

10.06

10.04

29.50 30.00 30.05 30.10 30.15 30.20 30.25
FREQUENCY ( kHz )

MAGNITUDE (Ω)

09915-005

CIRCUIT EVALUATION AND TEST

The schematic in Figure 1 was developed to improve impedance
measurement accuracy, and some example measurements were
taken. The AD8606 dual channel amplifier buffers the signal on
the transmit path and converts the receive signal from current
to voltage. For the three examples shown, the gain factor is
calculated for each frequency increment to remove frequency
dependent errors. A complete design package including schemat­ics, bill of materials, layout, and Gerber files is available for this
solution at http://www.analog.com/CN0217-DesignSupport.
The software used is the same software that is available with
evaluation boards and is accessible from the AD5933 and

AD5934 product pages.

Example 1: Low Impedance Range

Table 1. Low Impedance Range Setup for VDD = 3.3 V
Supply Voltage

Parameter Value

V p-p 1.98 V (Range 1)
Number of Settling Time Cycles 15
MCLK 16 MHz
R

20.1 Ω

CAL

RFB 20.0 Ω
Excitation Frequency Range 30 kHz to 30.2 kHz
Unknown Impedances R1 = 10.3 Ω,

R2 = 30.0 Ω,
C3 = 1 µF (Z
30 kHz)

The results of the low impedance measurements are shown in
Figure 3, Figure 4, and Figure 5. Figure 5 is for the 10.3 Ω
measurement and is shown on an expanded vertical scale.

The accuracy achieved is very much dependent on how large
the unknown impedance range is relative to the calibration
resistor, R
impedance of 10.3 Ω measured 10.13 Ω, an approximate 2%
error. Choosing an R
achieves a more accurate measurement; that is, the smaller the
unknown impedance range is centered around R
accurate the measurement. Consequently, for large unknown
impedance ranges, it is possible to switch in various R
resistors to break up the unknown impedance range using
external switches. The R
calibration during the R
switch to select various R
range of the signal seen by the ADC.

Also note that to achieve a wider range of measurements a
200 mV p-p range was used. If the unknown Z is a small range,
a larger output voltage range can be used to optimize the ADC
dynamic range.

. Therefore, in this example, the unknown

CAL

closer to the unknown impedance

CAL

error of the switch is removed by

ON

gain factor calculation. Using a

CAL

values can optimize the dynamic

FB

= 5.3 Ω at

C

is, the more

CAL

CAL

Figure 3. Measured Low Impedance Magnitude Results

Figure 4. Measured Low Impedance Phase Results

Figure 5. Measured 10.3 Ω Magnitude Results (Expanded Scale)

Rev. 0 | Page 3 of 6

CN-0217 Circuit Note

60

70

80

90

100

110

120

30 35 40

FREQUENCY (kHz)

IMPEDANCE MAGNITUDE (kΩ)

45 50

MEASURED

IDEAL

09915-006

–90.3

–90.2

–90.1

–90.0

–89.9

–89.8

–89.7

–89.6

–89.5

–89.4

–89.3

30 35 40

FREQUENCY (kHz)

PHASE (Degrees)

45 50

09915-007

30 35 40

FREQUENCY (kHz)

45 50

8160

8180

8200

8220

8240

8260

8280

IMPEDANCE MAGNITUDE (Ω)

R3

IDEAL

09915-008

30 35 40

FREQUENCY (kHz)

45 50

IMPEDANCE MAGNITUDE (kΩ)

R4

09915-009

213.5

214.0

214.5

21.50

215.5

216.0

216.5

217.0

217.5

218.0

218.5

IDEAL

Example 2: kΩ Impedance Range

Using an R
ances were measured according to the setup conditions listed
in Table 2. Figure 6 to Figure 10 document accuracy results.
To improve the overall accuracy, select an R
the unknown impedance. For example, in Figure 9, an R
closer to the Z
impedance range is large, use more than one R

Table 2. kΩ Impedance Range Setup for VDD = 3.3 V Supply
Voltage

Parameter Value

V p-p 0.198 V (Range 4)
Number of Settling Time Cycles 15
MCLK 16 MHz
R

99.85 kΩ

CAL

RFB 100 kΩ
Excitation Frequency Range 30 kHz to 50 kHz
Unknown Impedances R0 = 99.85 kΩ

of 99.85 kΩ, a wide range of unknown imped-

CAL

value closer to

CAL

value of 217.5 kΩ is required. If the unknown

C

resistor.

CAL

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 = 47pF (Z

= 84.6 kΩ at

C

40 kHz)

CAL

Figure 7. Phase Result for ZC = 47 pF, R

= 99.85 kΩ

CAL

Figure 6. Magnitude Result for Z

= 47 pF, R

C

CAL

= 99.85 kΩ

Rev. 0 | Page 4 of 6

Figure 8. Z

Figure 9. Z

= 8.21 kΩ, R

C

= 217.25 kΩ, R

C

= 99.85 kΩ

CAL

= 99.85 kΩ

CAL

Circuit Note CN-0217

0

20

40

60

80

100

120

30 32 34 36 38 40 42 44 46 48 50

IMPEDANCE MAGNITUDE (kΩ)

FREQUENCY (kHz)

R1

R3

C5

R0

C6

R2

09915-010

500

0

1000

1500

2000

2500

3000

3500

4000

4 24 44 64 84 104

IMPEDANCE MAGNITUDE (Ω)

FREQUENCY (kHz)

IDEAL
MEASURED

09915-011

4 24 44 64 84 104

FREQUENCY (kHz)

IDEAL
MEASURED

09915–012

–95

–90

–85

–80

–75

–70

–65

–60

PHASE (Degrees)

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 R

of 1 kΩ, an R of 10 kΩ, and a C of

CAL

10 nF, measured across a frequency range of 4 kHz to 100 kHz.
The magnitude and phase results versus ideal are plotted in
Figure 11 and Figure 12.

Table 3. R||C Impedance Range Setup for VDD = 3.3 V
Supply Voltage

Parameter Value

V p-p 0.383 V (Range 3)
Number of Settling Time Cycles 15
MCLK 16 MHz
R

1 kΩ

CAL

RFB 1 kΩ
Excitation Frequency Range 4 kHz to 100 kHz
Unknown Impedance R||C R = 10 kΩ

C = 10 nF

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

= 1 kΩ

CAL

Setup and Test

The EVAL-CN0217-EB1Z software is the same as that used on
the EVAL -AD5933EBZ application board. Please refer to the
technical note available on the CD provided for details on the
board setup. Note that there are alterations to the schematic.
Link connections on the EVAL-CN0217-EB1Z board are listed
below in Table 4. Also note that the location for RFB is located
at R3 on the evaluation board, and the location for Z

Table 4. Link Connections for EVAL-CN0217-EB1Z

Link Number Default Position

LK1 Open

LK2 Open

LK3 Open

LK4 Insert

LK5 Insert

LK6 A

COMMON VARIATIONS

Other op amps can be used in the circuit, such as the AD4528-1,

AD8628, AD8629, AD8605, and the AD8608.

Switching Options for System Applications

For this particular circuit, the Z
interchanged manually. However, in production, a low on­resistance switch should be used. 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 document use just one calibration resistor, and so a low
on-resistance switch such as the ADG849 can be used as shown

Rev. 0 | Page 5 of 6

in Figure 13. Multichannel switch solutions such as the quad

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

and R

UNKNOWN

CAL

= 1 kΩ

CAL

UNKN OWN

were

is C4.

CN-0217 Circuit Note

50kΩ

Z

UNKNOWN

R

CAL

S1

D

S2

R

FB

V

DD

IN

ADG849

50kΩ

A1

A2

09915–013

ADG812 can also be used. The errors caused by the switch

resistance on the Z
by choosing a very low R

are removed during calibration, but

UNKNOWN

switch, the effects can be further

ON

minimized.

LEARN MORE

CN-0217 Design Support Package:

http://www.analog.com/CN0217-DesignSupport

MT-085 Tutorial, "Fundamentals of Direct Digital Synthesis

(DDS)," Analog Devices.

Riordan, Liam, "AD5933 Evaluation Board Example

Measurement," AN-1053 Application Note, Analog Devices.

Buchanan, David, "Choosing DACs for Direct Digital

Synthesis," AN-237 Application Note, Analog Devices.

ADIsimDDS Design and Evaluation Tool

AD5933/AD5934 Demonstration and Design Tool

Data Sheets and Evaluation Boards

AD5933 Data Sheet

AD5933 Evaluation Board

AD5934 Data Sheet

AD5934 Evaluation Board

AD8606 Data Sheet

ADG849 Data Sheet

ADG812 Data Sheet

REVISION HISTORY

6/11—Revision 0: Initial Version

Figure 13. Switching Between R
UltraLow R

SPDT Switch (Simplified Schematic, All Connections and

ON

Decoupling Not Shown)

and Unknown Z Using the ADG849

CAL

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CN09915-0-6/11(0)

Rev. 0 | Page 6 of 6