8.2.1.1 Design Requirements

For this application, the design requirements are as provided in Table 8-1.

Table 8-1. Design Requirements

DESCRIPTION

VALUE

Battery voltage

VBAT = 48V

Supply voltage

VS = 5V

Full-scale range of ADC

VADC(fs) = VOUT = 5V

Quiescent current

125μA

ADC Resolution

8 bits

Effective number of bis (ENOB)

7.5 bits

Common-mode rejection ratio

80dB or a factor of 10000

8.2.1.2 Detailed Design Procedure

This section provides basic calculations for the INA600B difference amplifier with respect to the given design

requirements.

Firstly, the 48V battery voltage must be attenuated and interfaced to ADC reference voltage of 5V. This requires

a G = 1/10 or 0.10V/V and therefore the INA600B is chosen for the application.

Gain= VADCVBAT= 548isapproximately0.10

(1)

The maximum common-mode range of INA600 in a gain of 1/10 at 5V supply is given to be (V–) + 55V from the

INA600B Electrical Characteristics table.

This is well within the requirements for sensing 48V battery voltage and the common-mode rejection ratio

(CMRR) referred to output is a minimum of 95dB as per INA600B Electrical Characteristics table. This

corresponds to a attenuation factor of

156234.Thishelpsattenuatethe100mVcommon-modeerrorshownin

the Figure 8-2 to just under 2μV.

When referring to the INA600B output, Equation 2 calculates the common-mode error, RTO to approximately

2µV.

CMErr_RTO=100mV56231≅2μV

(2)

Next, INA600B has input impedance of 1.2MΩ as per the Electrical Characteristics table. Assuming a full battery

voltage of 48V, the input current through the resistor is calculated as:

IRIN= VBATRIN= 481.20M=40μA

(3)

This input current through the resistor adds to the amplifier quiescent current of 65μA resulting in a total current

consumption of 105μA, which meets the design requirement of 125μA.

Itotal=IRIN+IQ

(4)

The next step is to calculate the other error sources in the application. Maximum gain error and offset error as

per Electrical Characteristics table are 0.05% and 3.0mV for the Gain = 0.2V/V.

TotalError= 0.0005×482+0.00302=24.2mV

(5)

For an 8-bit, 5V ADC, VLSB is calculated as:

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