In general, the width of the stopband is between one and two decades, meaning that the highest attenuation is

between 10 to 100 around the middle frequency. This indication is commonly defined as the quality factor (Q

factor) of the filter and is calculated by:

Q=

1

(4)

The resistor is given by the internal resistor of 6 kΩ; therefore, the damping factor of the filter can further be

range of 100 µH.

9.1.2 Input Bias Current Return Path

The input impedance of the INA848 is extremely high (approximately 100 GΩ). However, a path must be

provided for the input bias current of both inputs. This input bias current is typically 25 nA. High input impedance

means that this input bias current changes little with varying input voltage.

Input circuitry must provide a path for this input bias current for proper operation. Figure 9-3 shows various

provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds

the common-mode range of the INA848, and the input amplifiers saturate. If the differential source resistance is

low, the bias current return path connects to one input (as shown in the thermocouple example in Figure 9-3).

With a higher source impedance, using two equal resistors provides a balanced input with possible advantages

of a lower input offset voltage as a result of bias current, and better high-frequency common-mode rejection.

Thermocouple

10 NŸ

Microphone,

Hydrophone,

and more

+

±

+

±

+

±

+

±

Transformer

AC Coupling

INA848

INA848

INA848

INA848

REF

REF

REF

REF

NOTE: Center tap in the transformer provides bias current return.

Figure 9-3. Providing an Input Common-Mode Current Path

INA848

SBOS946 – SEPTEMBER 2020

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