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).
Electronic filters
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:
  1. Active or inactive (18)
  2. Analogue or digital
  3. Linear or non-linear
  4. Infinite Impulse Response (IIR) or Finite Impulse Response (FIR) (19)
  5. 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.
Sending signals
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.
Device Evaluation
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}
Smartphone Application
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.