ANALOG DEVICES CN-0217 Service Manual

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

IMAGINARY

1024-POINT DFT

AGND DGND

ADC

(12 BITS)

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

OUT

VDD/2

1.48V

VOUT

RFB

VIN

47nF

20kΩ

50kΩ

A1, A2 ARE

AD8606

VDD/2

−

50kΩ

CN-0217

1.98V p-p

UNKNOWN

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

Rev.0

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atsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)

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09915-001

FREQUENCY (Hz)

100

100M10k 100k 1M 10M

= 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

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

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.

)

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