Typical Applications; Measuring Small Impedances - Analog Devices AD5934 Manual

TYPICAL APPLICATIONS

MEASURING SMALL IMPEDANCES

The AD5934 is capable of measuring impedance values up to
10 MΩ if the system gain settings are chosen correctly for the
impedance subrange of interest.
If the user places a small impedance value (≤500 Ω over the
sweep frequency of interest) between the VOUT and VIN pins,
it results in an increase in signal current flowing through the
impedance for a fixed excitation voltage in accordance with
Ohm's law. The output stage of the transmit side amplifier
available at the VOUT pin may not be able to provide the
required increase in current through the impedance. To have a
unity gain condition about the receive side I-V amplifier, the
user needs to have a similar small value of feedback resistance
for system calibration as outlined in the Gain Factor Setup
Configuration section. The voltage presented at the VIN pin is
hard biased at VDD/2 due to the virtual earth on the receive
side I-V amplifier. The increased current sink/source
requirement placed on the output of the receive side I-V
amplifier may also cause the amplifier to operate outside of the
linear region. This causes significant errors in subsequent
impedance measurements.
The value of the output series resistance, R
at the VOUT pin must be taken into account when measuring
small impedances (Z
UNKNOWN
the output series resistance is comparable to the value of the
impedance under test (Z
UNKNOWN
counted for in the system calibration (that is, the gain factor
calculation) when measuring small impedances, there is an
introduced error into any subsequent impedance measurement
that takes place. The introduced error depends on the relative
magnitude of the impedance being tested compared to the value
of the output series resistance.
TRANSMIT SIDE
OUTPUT AMPLIFIER
VOUT
R
OUT
DDS
RFB
PGA I-V
VDD/2
Figure 37. Additional External Amplifier Circuit for
Measuring Small Impedances
, (see Figure 37)
OUT
), specifically when the value of
). If the R value is unac-
OUT
2V p-p
R1
R2
V
DD
AD8531
AD820
20kΩ
AD8641
VDD/2
AD8627
20kΩ
1µF
R
FB
Z
VIN
UNKNOWN
The value of the output series resistance depends upon the
selected output excitation range at VOUT and has a tolerance
from device to device like all discrete resistors manufactured in
a silicon fabrication process. Typical values of the output series
resistance are outlined in Table 16.
Table 16. Output Series Resistance (R
Parameter
Range 1
Range 2
Range 3
Range 4
Therefore, to accurately calibrate the AD5934 to measure small
impedances, it is necessary to reduce the signal current by
attenuating the excitation voltage sufficiently and also account
for the R value and factor it into the gain factor calculation
OUT
(see the Gain Factor Calculation section).
Measuring the R
achieved by selecting the appropriate output excitation range at
VOUT and sinking and sourcing a known current at the pin
(for example, ±2 mA) and measuring the change in dc voltage.
The output series resistance can be calculated by measuring the
inverse of the slope (that is, 1/slope) of the resultant I-V plot.
A circuit that helps to minimize the effects of the issues
previously outlined is shown in Figure 37. The aim of this
circuit is to place the AD5934 system gain within its linear
range when measuring small impedances by using an additional
external amplifier circuit along the signal path. The external
amplifier attenuates the peak-to-peak excitation voltage at
VOUT by a suitable choice of resistors (R1 and R2), thereby
reducing the signal current flowing through the impedance and
minimizing the effect of the output series resistance in the
impedance calculations.
In the circuit shown in Figure 37, Z
output series resistance of the external amplifier which is
typically much less than 1 Ω with feedback applied depending
upon the op amp device used (for example, AD820, AD8641,
AD8531) as well as the load current, bandwidth, and gain.
The key point is that the output impedance of the external
amplifier in Figure 37 (which is also in series with Z
has a far less significant effect on gain factor calibration and
subsequent impedance readings in comparison to connecting
the small impedance directly to the VOUT pin (and directly in
series with R
OUT
impedance from the effects of R
output impedance in series with Z
Rev. A | Page 29 of 40
) vs. Excitation Range
OUT
Value (Typ) Output Series Resistance Value
2 V p-p 200 Ω typical
1 V p-p 2.4 kΩ typical
0.4 V p-p 1.0 kΩ typical
0.2 V p-p 600 Ω typical
value during device characterization is
OUT
UNKNOWN
). The external amplifier buffers the unknown
and introduces a smaller
OUT
.
UNKNOWN
AD5934
recognizes the
)
UNKNOWN