Operational Amplifier
The main task of an operational amplifier with AC number (AD620NA) is to
amplify low amplitude signals received from the abdomen so that the
signal can be used in the filter layer. Generally, operational
amplifiers are differential amplifiers with wide bandwidth, high input
impedance, stable and adjustable gain, and high common mode rejection
ratio. Furthermore, features include low noise, low input bias current
and low power of the AD620 make it suitable for medical applications and
instrumentation (17). This IC is an 8-pin
piece, with low power consumption, low offset, and high precision, which
has adjustable and high gain (figure
1,2).
They are electronic circuits that do signal processing, especially when
we want to eliminate unwanted signals from elements or increase them
(unwanted signals). Electronic filters can be as follows:
- Active or inactive (18)
- Analogue or digital
- Linear or non-linear
- Infinite Impulse Response (IIR) or Finite Impulse Response (FIR)
(19)
- Time discontinuity or time continuity
In this study, high-pass filter, active filters, Low Pass Filter with
amplification (LPF & AMP), and Notch Filter were used. Regarding the
frequency spectrum of EHG signal in the range of 0.1 to 3-5 Hz
(20), a high-pass filter of RC type with
a cutoff frequency of 0.1 Hz has been selected (figure 3,4).
For active filters, a combination
of active and inactive components (amplification) and an external power
source were used. Amplifiers that are frequently used in active filter
designs can have a high Q factor. They can reach resonance and this
occurs without using an inductor. However, the bandwidth of used
amplifiers limits their upper frequency limit
(21).
Low Pass Filter with
amplification (LPF & AMP) is to remove signals with a frequency higher
than 3 Hz. Due to the great advantage of the Sallen-Key type active
filter (22), the same type of filter with
amplification has been used in the design of the low-pass filter.
(figure 5).
Notch Filter can be used to
eliminate urban electricity artifact signals in this electrocardiogram
device. Because the urban electricity frequency is 50 Hz in Iran, we
must use a low-pass filter that is set to a frequency of 50 Hz to remove
the noise caused by urban electricity frequency. For this purpose, we
use a Twin-T Notch type filter and then an amplifier to amplify the
signal without noise. The designed filter is passive. Due to the loss of
another IC, the passive T-Notch filter effectively removes noise from
urban electricity.
Power supply
Based on our purpose, the device should be portable. So, a rechargeable
lithium battery was used. Due to the need for negative polarity in
powering operational amplifiers, IC 7660, which produces negative
voltage, has been used.
Because of the necessity for easy use of devices by pregnant women, data
must be transmitted wirelessly. The HC-05 data transfer module, which is
a Bluetooth to serial interface, is used for sending data to
smartphones.
At first, we evaluated the accuracy of the device with an ECG Signal
Simulator. ECG signals were entered into the device, and the device
output was recorded. The average of four repeated tests was calculated
in this section.
\begin{equation}
CV=\frac{\sigma}{\left|\mu\right|}*100\%\ that\ \sigma=\sqrt{\frac{1}{N}}\sum_{i=1}^{N}{({A_{\text{ti}}-\mu)}^{2}}\ and\ \mu=\ \frac{1}{N}\sum_{i=1}^{N}A_{\text{ti}}\backslash n\nonumber \\
\end{equation}The application was designed for patients with the aim of providing easy
access to uterine data and informing pregnant women about contractions.
Features of the application are determined with regard to the basic
needs of users. Data is saved on mobile phones. The application was
developed on Android.