Overview
Design Resources
Design & Integration File
 Schematic
 Bill of Materials
 Gerber Files
 PADS Files
 Assembly Drawing
Evaluation Hardware
Part Numbers with "Z" indicate RoHS Compliance. Boards checked are needed to evaluate this circuit.
 EVALAD5933EBZ ($71.53) High Accuracy Impedance Measurements Using 12Bit Impedance Converters
Device Drivers
Software such as C code and/or FPGA code, used to communicate with component's digital interface.
Features & Benefits
 High accuracy impedance measurement
 Complex impedance measurments using internal frequency generator
 Measure up to several hundred kohms
Product Categories
Markets and Technologies
Parts Used
Documentation & Resources

UG364: Evaluating the AD5933 1 MSPS, 12Bit Impedance Converter Network Analyzer2/27/2012PDF1202 kB

Fundamentals of Direct Digital Synthesis (DDS)2/14/2015PDF173 kB

CN0217: High Accuracy Impedance Measurements Using 12Bit Impedance Converters7/26/2011PDF253 kB
Circuit Function & Benefits
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.
Figure 1. Optimized Signal Chain for Impedance Measurement Accuracy (Simplified Schematic, All Connections and Decoupling Not Shown)
Circuit Description
Table 1. Output Series Resistance, ROUT, vs. Excitation Range for VDD = 3.3 V Supply Voltage
Range  Output Excitation Amplitude  Output Resistance, R_{OUT} 
Range 1  1.98V _{pp}  200 Ω typ 
Range 2  0.97V _{pp}  2.4 kΩ typ 
Range 3  0.383V _{pp}  1.0 kΩ typ 
Range 4  0.198V _{pp}  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 AV of 1 is less than 1 Ω up to 100 kHz, which is the maximum operating range of the AD5933/AD5934.
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 pp excitation voltage has a bias of 1.48 V. However, the currenttovoltage (IV) 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 highpass filter with a corner
frequency in the low Hz range. Removing the dc bias from the
transmit stage and rebiasing 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
Range  Output Excitation Amplitude  Output DC Bias Level 
1  1.98V _{pp}  1.48V 
2  0.97V _{pp}  0.76V 
3  0.383V _{pp}  0.31V 
4  0.198V _{pp}  0.173V 
Selecting an Optimized IV Buffer for the Receive Stage
The currenttovoltage (IV) amplifier stage of the AD5933/AD5934 can also add minor inaccuracies to the signal chain. The IV conversion stage is sensitive to the amplifier's bias current, offset voltage, and CMRR. By selecting the proper external discrete amplifier to perform the IV conversion, the user can choose an amplifier with lower bias current and offset voltage specifications along with excellent CMRR, making the IV conversion more accurate. The internal amplifier can then be configured as a simple inverting gain stage.
Selection of resistor R_{FB} 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 highpass filtered and rebiased
before buffering with a very low output impedance
amplifier. The IV 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, singlesupply capability, low bias current,
low offset voltage, and excellent CMRR performance. Some
suggested parts are the ADA45281, AD8628/AD8629, AD8605, and AD8606. Depending on board layout, use a singlechannel
or dualchannel amplifier. Use precision 0.1% resistors for both
the biasing resistors (50 kΩ) and gain resistors (20 kΩ and R_{FB})
to reduce inaccuracies.
Common Variations
Switching Options for System Applications
For this particular circuit, the Z_{UNKNOWN} and R_{CAL} were 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
onresistance 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 Z_{UNKNOWN} are removed during calibration, but by choosing a very low R_{ON} switch, the effects can be further minimized.
Figure 13. Switching Between R_{CAL} and Unknown Z Using the ADG849 UltraLow R_{ON} SPDT Switch (Simplified Schematic, All Connections and Decoupling Not Shown)
Circuit Evaluation & Test
Example 1: Low Impedance Range
Table 3. Low Impedance Range Setup for VDD = 3.3 V Supply Voltage
Parameter  Value 
V _{pp}  1.98V (Range 1) 
Number of Settling Time Cycles  15 
MCLK  16 MHz 
R_{CAL}  20.1Ω 
R_{FB}  20.0Ω 
Excitation Frequency Range  30 kHz to 30.2 kHz 
Unknown Impedances  R1 = 10.3Ω, R2 = 30.0Ω, C3 = 1 µF (Z_{C} = 5.3Ω< at 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_{CAL}. Therefore, in this example, the unknown impedance of 10.3 Ω measured 10.13 Ω, an approximate 2% error. Choosing an R_{CAL} closer to the unknown impedance achieves a more accurate measurement; that is, the smaller the unknown impedance range is centered around RCAL is, the more accurate the measurement. Consequently, for large unknown impedance ranges, it is possible to switch in various R_{CAL} resistors to break up the unknown impedance range using external switches. The R_{ON} error of the switch is removed by calibration during the R_{CAL} gain factor calculation. Using a switch to select various R_{FB} values can optimize the dynamic range of the signal seen by the ADC.
Also note that to achieve a wider range of measurements a 200 mV pp 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.
Figure 3. Measured Low Impedance Magnitude Results
Example 2: kΩ Impedance Range
Using an R_{CAL} of 99.85 kΩ, a wide range of unknown impedances 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_{CAL} value closer to the unknown impedance. For example, in Figure 9, an R_{CAL} closer to the Z_{C} value of 217.5 kΩ is required. If the unknown impedance range is large, use more than one R_{CAL} resistor.
Parameter  Value 
V _{pp}  0.198V (Range 4) 
Number of Settling Time Cycles  15 
MCLK  16 MHz 
R_{CAL}  99.85 kΩ 
R_{FB}  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 (Z_{C} = 26.5 kΩ at 40 kHz) C6 = 47pF (Z_{C} = 84.6 kΩ at 40 kHz) 
Figure 6. Magnitude Result for Z_{C} = 47 pF, R_{CAL} = 99.85 kΩ
Example 3: Parallel RC (RC) Measurement
An RC type measurement was also made using the configuration, using an R_{CAL} 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 versus ideal are plotted in Figure 11 and Figure 12.
Table 5. RC Impedance Range Setup for VDD = 3.3 V Supply Voltage
Parameter  Value 
V _{pp}  0.383V (Range 3) 
Number of Settling Time Cycles  15 
MCLK  16 MHz 
R_{CAL}  1 kΩ 
R_{FB}  1 kΩ 
Excitation Frequency Range  4 kHz to 100 kHz 
Unknown Impedance RC 
R = 10 kΩ C = 10 nF 
Figure 11. Magnitude Results for ZC = 10 kΩ10 nF, RCAL = 1 kΩ
Setup and Test
The evaluation software is that used on
the EVALAD5933EBZ 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 EVALAD5933EBZ 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_{UNKNOWN} is C4.
Table 6. Link Connections for EVALAD5933EBZ
Link Number  Default Position 
LK1  Open 
LK2  Open 
LK3  Insert 
LK4  Open 
LK5  Insert 
LK6  A 