ADA4930 analysis of a fully differential op amp (1)

The analog signal input of the ad conversion chip is fully differential, pseudo differential, single-ended input, where the fully differential input is the best, and at this stage the ADC converter in order to improve its performance, it is recommended that users use a fully differential input method. (AD7982, ADS8317, etc. can achieve the full differential input of the signal, figure 1 shows the application circuit of the AD7982, it is shown that the input is a fully differential input method), but the general sensor output signal is a single-ended signal, the full differential amplifier plays a key role.

Fig. 1 Application Circuit of AD7982

A fully differential amplifier (fully-differential) is a chip that is used to convert a single-ended signal to a differential signal, or to convert a differential signal to a differential signal. An example of Adi's ada4940-1 is used to analyze how the fully differential amplifier works as a single-ended-to-differential signal and the calculation of external resistance parameters.

Fig. 2 Application Circuit of ada4940-1

Figure 2 shows the application circuit of the ada4940-1, as described in its datasheet, VOCM is expressed as the output signal of the common-mode voltage, the size of the external input is determined by the input signal is independent of the common-mode voltage. There are two closed loops in the circuit, the upper and lower symmetry, in order to make the closed-loop performance consistent, two closed-loop parameters should be consistent.

　　The following focuses on the implementation of ADA4940 as a single-ended-to-differential signal processing:

Fig. 3 circuit diagram of ADA4940 as single-ended-to-differential

Figure 4 Analysis of single-ended-to-differential signal 1

Fig. 4 is the input resistor of the single-ended-to-differential signal given in the datasheet, and the input resistance of the positive input of the amplifier is about 1.33kω in the rf=rg=1kω circuit, and the derivation process is not given in datasheet.

My derivation process is as follows:

Figure 5 Analysis of single-ended-to-differential signal 2

5 is shown below:

Assuming positive input vin=v, the resulting current I, the negative input terminating GND

The differential signal vin_dm=v at the input, and in this circuit, rf=rg=1k, the differential signal vout_dm=vin_dm* (RF/RG)of the output, see the data sheet for the formula.

Then positive output: vout+=vocm+1/(2*v)-------------------(1)

Simultaneous negative output: vout-=vocm-1/(2*v)----------------------(2)

Where the VOCM is the output common-mode voltage, introduced by the external pin, and the input common-mode voltage independent, so that the user can set the desired common-mode voltage within a certain range.

Can get:

I= (VIN-VP)/rg---------------------------------------------------(3)

vp=vn=rg* (vout+)/(RG+RF)---------------------------------(4)------resistor voltage Divider

Combine (1) ~ (4) to get:

i={vin-(rg/(RG+RF)) *vin-rg*vocm/(RG+RF)}/rg-------(5)

The input signal v produces a ΔV change, which is:

Δi= (δvin-(rg/(RG+RF)) *δvin)/rg---------------------------(6)

By the formula (6) can be obtained, the input resistance of the circuit Rin can be expressed as:

rin=δvin/δi=rg/(1-RG/2 (RG+RF))-------------------------------(7)

Brought into the rf=rf=1kω, you can get:

rin=1.333kω.

Reference:

Http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4940-1_4940-2.pdf

2016-12-28

16:15:30

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Full differential op amp ADA4930 analysis (1)