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Tomorrow, Let’s take a look at the third and fourth posts in this series of blogs – Coin-phase requirements and configurations and Co-phase requirements and configurations that support attenuation.
In-phase requirements and equipment Jamaica Sugar
Bipolar electronic signals swing up and down at low voltage (0 V). Since the ADAQ798x integrated ADC can only convert electronic signals from 0 V to VREF, the bipolar electronic signals need to be DC biased and appropriately adjusted for the ADC. To accomplish this task, the following configuration adds two resistors (R1 and R2Jamaica Sugar) to the standard non-inverting configuration.
This device will output electronic signals and Jamaica SugarA separate DC voltage sum to bias the ADC driver input to the ADC center level output (VREF/2), thereby completing bipolar to unipolar conversion. The reference voltage (VREF) is used It is often possible to use a DC voltage, which eliminates the need for additional circuitry (the ADAQ798x always comes with a voltage reference anyway!). It also prevents VREF errors from adding offset errors to the system, since the DC bias of the ADC driver is always VREF. half. For these reasons, we will specifically discuss a device that uses VREF as a DC “conversion” voltage.
The transfer function for this device is as follows:
Similar to ordinary in-phase devices, the ratio of Rf and Rg determines the change from IN+ to AMP_OUT gain, but this ratio now also depends on the output amplitude of vIN. Note that vIN is bipolar, but the voltage on the non-inverting node is unipolar. This means that for the minimum value of vIN, the voltage on IN+ must be 0 V. ：
From this relationship, R1 and R2 ratio:
Rf and Rg can be determined using the transfer function of the device and the condition that the ADC driver output (vAMP_OUT) is equal to VREF/2 when vIN is 0 V. Solving the equations for Rf and Rg gives:
Now we have R1 and R2As for the ratio of Rf and Rg, but we also need to choose a specific value. We have discussed the choice of Rf and Rg values in this blog series, Increasing Gain on Unipolar Outputs. The selection of R1 and R2 should be based on the noise, accuracy, and output impedance requirements of the application. A small resistor will improve the noise and reduce the offset error caused by its interaction with the ADC driver output bias current, but to increase the output impedance and reduce the input current of the reference source, a large resistor is required. The output impedance of this circuit is:
Note, For the special case of vIN amplitude ±VREF, the ratio of Rf and Rg is 0. In this case, the ADC driver gain is 1, which means that Rg is omitted and Rf can be considered 0 Ω.
For example, ADAQ7980 needs to perform bipolar to unipolar conversion on a ±1 V output electronic signal, VREF = 5 V, use Rf = 2 kΩ. Applying the above formula, R2 must be 5 times R1 and Rf must be 2 times Rg. JM EscortsRf is 2 kΩ, so Jamaica Sugar DaddyRg must be 1 kΩ. The specific values of R1 and R2 can be selected according to the application requirements. For this example, we hope to choose a combination of R1 and R2 to offset the impact of the output bias current on the offset error. As explained in the technical article “Optical Amplifier Output Bias Current”, to achieve this goal, R1||R2 should be equal to Rf||Rg, so R1= 800 Ω and R2= 4 kΩ.
Let’s Jamaicans Sugardaddy consider another example: vIN= ±10 V, VREF= 5 V. In this case, we will encounter the problem that the ratio of Rf and Rg is positive, so this output range cannot actually be achieved using this configuration. In fact, the maximum vIN suitable for this configuration is ±VREF, where the ADC driver gain is equal to 1. Fortunately, we will discuss two other devices that allow us to exceed this output range in the next article in this series.
If R2 is connected to ground instead of VREF, the above configuration can also be used for unipolar electronic signals. This correction requires attenuation for unipolar output electronics for ADCsJM Escorts signal (amplitude greater than VREF) is valid. In this case, the ADC driver is most likely unit gain, so Rf and Rg are not required.
As above As mentioned, if the application requires high output impedance, R1 and R2 must be large, which may increase the noise floor of the system. We can compensate for the noise increase by increasing the shunt capacitance and/or through oversampling and decimation. Both schemes reduce the noise floor by losing the output electronic signal bandwidth. However, for low bandwidth or DC applications, the output bandwidth is less important, so these devices are more suitable for low bandwidth, high output impedance applications. This topic will be discussed in more detail in the next article.
However, one issue that is not covered is the output of the ADC driver through the resistor. >Jamaica SugarThe larger the resistance, the greater the DC error caused by adjusting the ratio of R1 and R2 to compensate for the unnecessary voltage drop. Select JM Escorts to select the values of Rf and Rg to offset the offset caused by R1 and R2, which can reduce this error, but the output range will There is a loss. However, it should be noted that Rf must be small enough to ensure that the amplifier is stable, so the second solution is not always feasible
Support attenuation of the in-phase requirement and configuration
For greater than ±VREF. For electronic signals, the following configuration can be used to perform attenuated bipolar to unipolar conversion
This configuration is similar to the configuration discussed in the [Co-phase requirements and configuration] above. The difference is that Rf and Rg are no longer required. But R3 is added to provide additional electronic signal attenuation. The transfer function of this deviceJM Escorts is as follows:
This time the mathematics of finding the ratio of R1, R2 and R3 The calculation is more complicated, but we can find the ratio of the resistors in a similar way to the previous configuration.Then, you can select specific values based on your application needs. For the sake of brevity, we will not describe every step of the derivation here JM Escorts, but we will see that for the minimum and maximum values of vIN For large values, a simplification of the transfer function allows us to derive the resistance ratio.
The ratio of R1 and R2 is obtained by using the transfer function of the device and substituting in the minimum value of vIN (such that vAMP_OUT equals 0 V):
R3 is not present in the formula. Solve R1 and R2 to get:
JM EscortsThe ratio of R1 and R3 is obtained by substituting the maximum value of vIN (making vAMP_OUT equal to VREF):
This time, R2 is not presented. Solve R1 and R3 to obtain:
At this point, we can choose the value of any of the resistors (considering the VREF and vIN ranges), and then calculate the other two value of a resistor. As before, the important measurements are output impedance versus system noise and offset error. The output impedance (ZIN) of this circuit is:
Consider again the example from the [Non-Phase Requirements and Configuration] section above, where vIN= ±10 V, VREF= 5 V, and an output impedance of 1 MΩ designThe configuration. For this combination of vIN and VREF, R1 must be twice R2 and equal to R3. Using the ratio of R2 and R3 to R1 to the output impedance formula gives R1 = 750 kΩ. Therefore, R2 and R3 are 3Jamaicans Sugardaddy75 kΩ and 750 kΩ respectively.
As mentioned above [in-phase requirements and configuration], the output impedance and system noise performance need to be measured. Achieving high output impedance requires a large resistor, which generates more thermal noise and interacts with the output current noise of the ADC driver to generate more output voltage noise. Both will increase the effective rms voltage noise at the ADC output, resulting in a significant performance drop. In the above example, the total system noise is approximately 334 μV rms (when using a 5 V reference source Jamaica Sugar, the static range is reduced by a full 15.5 dB , dropped from 92 dB to 74.5 dB)!
But there is hope! This configuration can actually achieve near-optimal performance if the output bandwidth is limited. For example, if the output bandwidth in the above example is limited to 20 kHz, the overall system noise drops almost 10 times to 48 μV rms (for VREF = 5 V, the static range is 91.4 dB)! We can limit the output bandwidth (BWin) by increasing the shunt capacitor CS, as shown in the figure below. Note that for these noise calculations, we can think of R1, R2, and R3 as a single resistor, RS, where RS is the parallel combination of R1, R2, and R3.
Skills Guide “Operation Reduction of Single Vertex System” Total Device Input Noise Calculation explains how to calculate the Jamaicans Sugardaddy noise generated by RS (including thermal noise and its interaction with the ADC driver output current Probation). The important difference with the ADAQ798x is that the noise bandwidth is set by the integrated RC filter rather than the reducer bandwidth as in the guide. The useful value noise added by RS to the ADC output is:
(en is the Johnson noise of RS, and G is the ADC driver gain.)
CS reduces the noise reaching the ADC by reducing the output bandwidth of the ADC driver. If RS and Jamaica Sugar DaddyThe cutoff frequency of CS is much smaller than the cutoff frequency of the integrated RC filter (4.42 MHz), so the noise contribution of RS can be calculated using RS and CS, replacing R and C in the above formula.
The total system noise is the root sum of each noise source in the ADAQ798x, including the noise of RS, the output voltage noise of the ADC driver, and the effective value noise of the ADC. The figure below shows the system noise and output corresponding to multiple RS values.
Pay attention to the output. The bandwidth decreases, and the overall system noise trend is Jamaicans EscortThe total useful value of ADAQ798xJM Escortsnoise (44.4 μV rms). This means that the noise reduction benefits achieved by decreasing the bandwidth will increase at a certain frequency, Jamaicans SugardaddyThe frequency depends on the effective value of RS.
This section discusses an ADC driver configuration that allows the ADAQ798x to accept bipolar outputs greater than ±VREF and explains how to The resistor value (and optional shunt capacitor CS) accounts for the output impedance and system noise.
While adding CS has been shown to reduce noise, it also limits the available output bandwidth for this configuration. It is often unrealistic to achieve high output impedance when using a wide network bandwidth. This configuration is only recommended for low bandwidth applications that require high output impedance.
Original title: Engineer Blog丨Universal ADC, how you should use it (in series)
Article source: [Microelectronic signals: analog_devices, WeChat official account: analog_devices] Welcome to add follow-up attention! Please indicate the source when the article is transcribed and published! .
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