MAXREFDES73 Datasheet, PDF (5/17 Pages) Maxim Integrated Products – Single-chip AC impedance measurement

Figure 2. MAXREFDES73# AC impedance measurement block diagram.
The sample under test (e.g., wrist skin) is used as a part of the input impedance of an
inverting amplifier. The circuit is stimulated with a coherent sinusoidal input. The response
of the circuit is coherently detected with a digital baseband quadrature sampling receiver
to detect the amplitude and phase of the network response. A known calibration path is
also measured. From these two responses, the complex impedance of the sample under
test can be determined.
From the ratio of the responses of the test load to the calibration path, the complex
impedance can be accurately determined from a low power observation. To determine the
response of each path, a minimum of two ADC samples is required.
To measure the impedance at a desired frequency (Fc), a sinusoidal voltage is applied to
the test load:
x(t) = cos(2π × Fc × t)
The output, observed at the ADC, is a scaled and phase shifted version of the input:
y(t) = V_L × cos(2π Fc × t + θ)
To extract the phase of the received signal, coherent detection is required. Digital
baseband quadrature sampling can easily be implemented if the ADC sampling can by
synchronized with the DAC output (AC excitation). The key feature of this solution is that
the ADC sample rate is 4 times the excitation frequency, as illustrated in Figure 3.