Design Details

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Hall Effect Sensor
The Solenoid
Gain Stage

This page contains schematic details of my thesis. For a general background information behind the thesis, please see the Background page. Although it is not necessary, reading the Background page makes it easier to understand the design details described on page.

Hall sensor based electric current sensor is composed of many parts. The sensor contains a solenoid, a Hall effect sensor, and the gain stage. The final current reading can be performed by a variety of voltmeters, digital multimeters(DMM) or even fancy testers. The four elements are used to make the current sensor (See figure 1). The solenoid converts the electric current to be sensed in to a magnetic field. The magnetic field is converted to a voltage via the Hall effect sensor. The hall voltage is amplified and conditioned with the gain stage. The final readout is performed by the DMM or tester. Notice that the gain stage is split between the integrated circuit and the PCB. The reasoning behind that design decision is discussed in the Gain Stage.

System Requirements

Before we perform a system anaylsis and determine the required specifications for the individual blocks, let's examine the goals of the system:

It must do the following :

	Senses electric current and read it back as a large voltage signal that a DMM or tester can use.
	Minimum detectable current = 1mA
	Maximum detectable current = >200mA
	Low input impedence electric current inputs - < 0.25 ohms.
	Uses standard CMOS technology with p- substrate (no special masks or features).
	Accuracy must be within 1mA after calibration (calibration method must be fast - less than 1ms).
	Sensor must be testable.
	Gain = 1V/1A
	Power supply tolerance is 10% with 3.3V rail.
	Design runs off of 3.3V power supply
	Sensing bandwidth 10kHz

Items that would be nice to have, but not necessary :

	The sensor should be easily adaptable to IEEE 1149.4 analog test bus.
	The sensor should consume low power (< 5mW) and dimensionally small (<1000um^2).
	The sensor's measurement bandwidth should be as large as possible (>1kHz).

Additional requirements that have been placed:

	Design to 0.25um or 0.18um -- which ever process is available at CMC at the time. Minimum L is 0.35um to ensure easy porting between processes.
	DMM/Tester sensitivity should be below 1mV.

Pinout Diagram

System Block Diagram

Pin Description

Pin Name	Direction	Description
ENABLE	CMOS IN	The ENABLE pin can bring the block in to a low power state and cause the output (OUT) to be high-impedance. The ENABLE is asserted, the block operates normally and measures current from IMEASIN/IMEASOUT
MODE[3:0]	CMOS IN	The MODE pins select the appropriate state within the block. It also controls the state of OUT and what state the Hall sensor is in.

IMEASIN	Analog IN	The current that is being measured. Current that flows in to this node will be observed as a positive output on the block. If the current flow is reversed (i.e. current flows out of IMEASIN), then the block's output will be negative.
IMEASOUT	Analog OUT	The current that is being measured.
ISRCDC	Analog IN	1mA constant current source is supplied to the ISRCDC pin.

OUT	Analog OUT	The voltage output of the block. It is output is dependent on the MODE and ENABLE bits. When ENABLE is high, the OUT is tri-state or hi-impedance.

GND	Ground	3.3V ground
VDD	Power 3.3V	3.3V power supply

Block Diagram Description

Gain Block Calculations

With those points in mind, let's tackle the 1V/1A sensitivity issue. This means that at minimum 1mA current would produce a 1mV signal -- most test equipment should be able to sense this signal. The following table contains typical values of gains for the solenoid, hall sensor. It also shows the required gain.

Solenoid (Single Loop)	15 mT/A
Hall Sensor	0.237 V/T
Total Sensor Gain	3.56 mV/A

Required Gain	1 V/A
Total Sensor Gain	3.56 mV/A
Gain Block Amplification Factor	281 V/V or ~50 dB

Note that the Hall sensor and Solenoid gain parameters were estimated from past papers and from classical magnetic theory calculations (i.e Maxwell Equations).

Noise Calculations.

Since the most of the sensors generate a very low signal strength, noise from transistors and resistors become an important fact. From our requirements, we require 1mA of accuracy. We shall assume the Solenoid is noiseless since the series resistance is low (<0.25 ohms) and reactive elements is considered noiseless. The Hall sensor has a source resistance of 1kohm; thus, the Hall sensor will have thermal noise contribution from the source resistance. The gain stage will also contribute noise in the form of noise figure (NF).

The following table is the noise power budget calculations (F = 10kHz, T= 300K):

Minimum Accuracy @ 1mA	1 mV or -60dB

Noise from Solenoid	0 dB
Noise from Hall Sensor (1 kOhm)	-127.82 dB
Noise Figure from Gain Stage	10 dB
Gain from Gain Stage	50 dB
Final Noise Output	-67.82 dB

Noise Margin	7

From the table above, there is a 5.82dB margin for noise. The margin should be sufficient to other minor factors.