LM7705 datasheet(13/22 Pages) TI

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RF1

RG1

RF2

RG2

ADC

VREF

GAIN = 10x

GAIN = 5x

ADC122S021

1/2 LMP7702

SENSOR

LM7705

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SNVS420B – NOVEMBER 2008 – REVISED MARCH 2013

The sensor has a DC output signal that is amplified by the op amp. For an optimal signal-to-noise ratio, the

output voltage swing of the op amp should be matched to the input voltage range of the Analog to Digital

Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However,

the low side of the range can’t be adjusted and is affected by the output swing of the op amp.

Example:

Assume the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7701, using a

0/+5V supply voltage, having an output drive of 50 mV from both rails. This results in an output range of 50 mV

to 4.95V.

Let choose two resistors values for RG1 and RF1 that result in a gain of 50x. The output of the LMP7701 should

swing from 0 mV to 4.5V. The higher value is no problem, however the lower swing is limited by the output of the

LM7701 and won’t go below 50 mV instead of the desired 0V, causing a non-linearity in the sensor reading.

When using a 12 bit ADC, and a reference voltage of 5 Volt (having an ADC step size of approximate 1.2 mV),

the output saturation results in a loss of the lower 40 quantization levels of the ADCs dynamic range.

Two-Stage, Single Supply True Zero Amplifier

This sensor application produces a DC signal, amplified by a two cascaded op amps, having a single supply. The

output voltage of the second op amp is converted to the digital domain. Figure 30 shows the basic setup of this

application.

Figure 30. Sensor with DC Output and a 2-Stage, Single Supply Op Amp.

The sensor generates a DC output signal. In this case, a DC coupled, 2-stage amplifier is used. The output

voltage swing of the second op amp should me matched to the input voltage range of the Analog to Digital

Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However,

the low side of the range can’t be adjusted and is affected by the output drive of the op amp.

Example:

Assume; the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7702 (Dual

LMP7701 op amp) that can be used for A1 and A2. The op amp is using a 0/+5V supply voltage, having an output

drive of 50mV from both rails. This results in an output range of 50 mV to 4.95V for each individual amplifier.

Let choose two resistors values for RG1 and RF1 that result in a gain of 10x for the first stage (A1) and a gain of

5x for the second stage (A2) The output of the A2 in the LMP7702 should swing from 0V to 4.5 Volt. This swing is

limited by the 2 different factors:

1. The high voltage swing is no problem; however the low voltage swing is limited by the output saturation

voltage of A2 from the LM7702 and won’t go below 50mV instead of the desired 0V.

2. Another effect has more impact. The output saturation voltage of the first stage will cause an offset for the

input of the second stage. This offset of A1 is amplified by the gain of the second stage (10x in this example),

resulting in an output offset voltage of 500mV. This is significantly more that the 50 mV (VDSAT) of A2.

When using a 12 bit ADC, and a reference voltage of 5 Volt (having an ADC step size of approximate 1.2 mV),

the output saturation results in a loss of the lower 400 quantization levels of the ADCs dynamic range. This will

cause a major non-linearity in the sensor reading.

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