ADA4351-2ACPZ-R7 datasheet(24/36 Pages) AD

24 / 36 page

Data Sheet

ADA4351-2

THEORY OF OPERATION

analog.com

Rev. 0 | 24 of 36

For example, a 16-bit ADC with a 4.5 V reference has a 68.7 µV

step size. If RSH = 10 MΩ, for RF = 1 MΩ, the output-referred

offset is 100 µV × (1 + 1 MΩ/10 MΩ) = 110 µV, while the same

circuit with an RF = 1 kΩ has a 25°C maximum 100.01 µV of

output-referred offset. In both cases, the output-referred offset is

less than 2 codes indicating that the output error due to the offset

voltage is relatively fixed vs. the RF value. The ±0.85 µV/°C (−40°C

to +125°C) maximum leads to a 140 µV shift maximum over the full

−40°C to +125°C span, or a worst case of 2 codes.

Input Bias Current

Input bias current (IB) adds to the total output DC error if the

TIA gain (RF ) or +IN source resistance (RS) are large enough to

exceed this relatively small input offset voltage related error. These

input bias currents increase exponentially with TJ where operating

at <105°C is a typical constraint for improved accuracy. On the

inverting input, IB adds with switch off leakage current (IOFF) from

the inactive channel, refer to Figure 79, summing back from the

inactive feedback channel into the active channel. If the inactive

channel does not have a feedback resistor installed, ignore this

additive term. Adding the maximum ±50 pA switch off leakage to

the ±70 pA inverting input bias current (−40°C to +105°C) adds an

output error greater than the 100 µV maximum offset voltage error

for RF values > 100 µV/120 pA = 883 kΩ.

Figure 79. Error Currents in Switched Gain Transimpedance Amplifier

Similarly, the input bias current from the noninverting input (max-

imum 110 pA from −40°C to +125°C) has negligible impact on

the TIA circuit if the bias current induces error less than the 100

µV offset voltage error, which occurs when the sensor output

impedance (RS) is less than 100 µV/110 pA = 909 kΩ. When using

the ADA4351-2 to measure a high impedance voltage sensor at

the noninverting input, as shown in Figure 80, this may not be the

case. Instead, the input bias current results in a voltage error at the

noninverting input equal to the sensor output impedance times the

bias current. This voltage error adds to the input offset voltage error

and is gained up to the output by the noninverting signal gain of the

circuit (1 + RF/gain resistor (RG)).

Note that the contribution of the input bias current to the DC error

is normally a trivial design constraint, and in many cases, is usually

dominated by that of the offset voltage.

Figure 80. Using the ADA4351-2 to Measure a High Impedance Voltage

Output Sensor at the Noninverting Input

Typically for an amplifier with CMOS input devices, the input bias

current is dominated by the electrostatic discharge (ESD) protection

diodes. For the ADA4351-2, the input protection diodes are boot-

strapped to reduce the reverse-biased leakage of these protection

diodes that results in the specified low input bias current, as shown

in Figure 81.

Figure 81. Using Bootstrapped ESD Protection Diodes to Reduce Input Bias

Current

As temperature increases, the input bias current due to the ESD

protection diodes increases exponentially. The input bias current

for the inverting input ADA4351-2 is <70 pA up to 105°C at a 5

V supply. Using a 100 µV maximum input offset voltage, this error

term is less than VOS for RF < 1.42 MΩ. This maximum input bias

current increases to 100 pA over the full −40°C to 125°C range

adding more error to the output voltage.

ADA4351-2ACPZ-R7 数据表(PDF) 24 Page - Analog Devices

ADA4351-2ACPZ-R7 数据表(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

数据表下载