8.1.1 Using an Operational Amplifier Between Multiplexer Output and ADC Input

Adding an operational amplifier provides a high input impedance to the sensor source and buffers the capacitive ADC input from high-impedance sensor circuits, as shown in Figure 57. High-impedance input signals can be momentarily disrupted when coupled directly to a capacitive input like that of a sampling ADC. This disruption can create errors when sampling. The use of an operational amplifier is recommended in these cases.

Figure 57. Sensor Data Acquisition With TLV2780 Buffer Amplifier

8.1.2 Using an Operational Amplifier and RC Filter Between Multiplexer Output and ADC Input

Placing an RC low-pass filter in the signal path allows for filtering out noise. The RC component values should allow for sufficient settling time when changing from channel to channel. The time required for a full-scale input signal to settle to within 1LSB of a 12-bit ADC is given by Equation 3:

R_X and C form a low-pass filter for removing sensor and noise from other sources at the operational amplifier input pin. The low-pass bandwidth is given by Equation 4:

The f_–3dB should be chosen so that the signals of interest are within half of the programmable sampling frequency. The noise bandwidth is given by Equation 5:

This term should be set to reduce noise bandwidth but still allow for enough settling time. The ADS7924 has internal registers ACQCONFIG (address = 14h), PWRCONF (address = 15h), and SLPCONFIG (address = 13h) that can be programmed to slow down the channel-to-channel power up, acquisition, and sleep periods if needed to allow for a longer settling time requirement.

In Figure 58, R is the sum of the sensor output impedance R_SENSOR, the internal MUX resistance R_MUX (approximately 60 Ω), and external resistor R_X. The primary benefit of having the filter at the input of the operational amplifier is that the amplifier does not have to drive the filter, which can cause instability with large capacitor values that may be needed to filter noise to low levels.

The TLV2780 typically powers up from a shutdown state in 800 ns. This period is well within the ADS7924 minimum acquisition time of 6μs. Setting the PWRCONFIG register (address = 15h) allows for more time if another operational amplifier with a shutdown feature is used.

NOTE: f_–3dB BW = 159 kHz, R = 1 kΩ, and C = 1 nF where R = R_MUX + R_SENSOR + R_X.

Figure 58. Sensor Data Acquisition With Filter and TLV2780 Buffer Amplifier

8.1.3 Using an RC Filter Between Multiplexer Output and ADC Input

For applications where low-output impedance signals are provided for the ADS7924 inputs, a simple RC filter may suffice, as shown in Figure 59.

NOTE: f_–3dB BW = 159 kHz, R = 1 kΩ, and C = 1 nF where R = R_MUX + R_SENSOR + R_X, C = C_X + C_ADCIN, R_MUX is approximately 60 Ω, and C_ADCIN is approximately 15 pF.

Figure 59. Sensor Data Acquisition With Filter Only

C_X should be greater than 200 pF, if possible. When coupled directly to the ADC input, using a capacitor with this value allows for faster settling when scanning between channels.

8.1.4 Operational Amplifier With Filter and Gain Option Between Multiplexer Output and ADC Input

Both filtering and gain are added in Figure 60. Gain is given by Equation 6:

NOTE:

f_–3dB BW = 159 kHz, R = 1 kΩ, and C = 1 nF where R = R_MUX + R_SENSOR + R_X, and R_MUX is approximately 60 Ω. Gain = 1 + R₁ / R₂.

Figure 60. Sensor Data Acquisition With Gain Set Resistors, Filter, and TLV2780 Buffer Amplifier

8.1.5 Driving an RC Filter With an Operational Amplifier Between Multiplexer Output and ADC Input

A filter can be placed at the output of the operational amplifier, as shown in Figure 61. Ensure that the operational amplifier is capable of driving the RC filter circuit without the operational amplifier becoming unstable. One of the benefits of this circuit is that the operational amplifier noise is filtered along with sensor and other system noise right at the ADC input pin.

NOTE:

C = 200 pF, R = 1 kΩ, and the capacitance at the ADCIN pin is approximately 15 pF.

Figure 61. Sensor Data Acquisition With an Operational Amplifier Driving an RC Filter

8.1.6 Average Power Consumption

With its fast conversion time and programmable sleep time with near-zero power, the ADS7924 allows periodic monitoring of the inputs with a very low average power dissipation, especially as the monitoring interval increases. The average current required can be calculated as the weighed average of the currents consumed during the power up, acquisition, converting, and sleep periods using Equation 7.

Equation 7.

As an example, calculate the average current in the following configuration:

• Mode programmed to Auto-Scan with Sleep
• Power-up time (t_PU) programmed to '0'
• Acquisition time (t_ACQ) programmed to 6 μs
• Sleep time (t_SLEEP) programmed to 2.5 ms
• AVDD = 2.2 V

Looking at Figure 27, the cycle time is seen to equal t_CYCLE = 4t_PU + 4t_ACQ + 4t_CONV + 4t_SLEEP = 4(0) + 4(6 μs) + 4(4 μs) + 4(2.5 ms) = 10.04 ms.

Table 5 lists the supply current for different supply voltages and operating conditions. Using the data for 2.2 V with the calculated cycle time in Equation 7 gives the following average current:

Equation 8.

The acquisition, conversion, and sleep times are multiplied by 4 because these are repeated four times in one cycle when in auto-scan with sleep mode.

Average power dissipation for the previous configuration where all four inputs are monitored every 10 ms is (2.2 V)(2.5 μA) = 5.5 μW.

Figure 3 and Figure 4 plot Equation 7 to help illustrate the relationship between cycle time and average power dissipation.

8.2.1 Design Requirements

Table 6 shows the design parameters for this typical application.

8.2.2 Detailed Design Procedure

8.2.2.2 Selecting the Operational Amplifier

The key parameters for selecting the operational amplifier for this circuit are noise, offset voltage and input bias current. The offset voltage and input bias current affect the DC accuracy whereas the noise of the amplifier increases the total noise at the input of ADC, the total noise at the input of ADC (V_n) can be calculated by Equation 9. V_n must be less than 200-µV RMS for this circuit design.

Equation 9.

where

• e_{n_RMS} is the input voltage noise density of the amplifier.
• V_{N_ADC} is the DC noise of the ADC. For ADS7924 , DC Noise is specified as 0.125 LSB RMS.
• V _{1/f_AMP_PP} is the peak to peak low-frequency noise at the input of amplifier.
• f-_3dB is the bandwidth of RC filter at the output of amplifier.

OPA313 is selected for this design for its low noise (25 nv/√Hz), low offset voltage (0.5 mV) and low input bias current (0.2 pA).

8.2.3 Application Curves

Max Error = -0.0035%		Min Error = -0.067%

Figure 63. Error vs Input Voltage

Sigma = 0.05

Noise = 61µV RMS

Figure 64. Number of Hits vs Output Code