Datasheet LT1635 (Analog Devices) - 9

LT1635
U U W U APPLICATIO S I FOR ATIO
The LT1635 is fully specified with V + = 5V, V – = 0V and should be taken to keep the output from saturating. For VCM = 2.5V. The op amp offset voltage is internally example, a 1mV input signal will cause the amplifier to set trimmed to a minimum value at these supply voltages. A up in its linear region in the gain 100 configuration as unique feature of this device is that it operates from a shown in Figure 2a. However, 1mV is not enough to make single 1.2V supply up to ±5V. A full set of specifications is the amplifier function properly in the voltage follower mode provided at ±5V supply voltages. The positive supply pin (Figure 2b). of the LT1635 should be bypassed with a small capacitor (about 0.1µF), as well as the negative supply pin when 99R using split supplies. 5V 5V R – –
Op Amp
OUTPUT LT1635 LT1635 100mV SATURATED OP AMP OP AMP The LT1635 is fully specified for single supply operation, + + ≈2mV 1mV 1mV i.e., when the negative supply is 0V. Input common mode 1635 F02 range of the op amp includes ground and the output
(a) (b)
swings within a few millivolts of ground while sinking current. The input stage of the op amp incorporates phase
Figure 2. Gain 100 Amplifier and Voltage Follower
reversal protection to prevent false outputs from occur- ring when the input is below the negative supply. Protec-
Distortion
tive resistors have been included in the input leads so that There are two main contributors of distortion in op amps: current does not become excessive when the inputs are distortion caused by nonlinear common mode rejection forced below the negative supply. and output crossover distortion as the output transitions The op amp also includes an offset nulling feature, this is from sourcing to sinking current. The common mode accomplished by connecting the BALANCE pin (Pin 5) to rejection ratio of the LT1635 is very good, typically 110dB. a variable voltage derived from the reference output. The Therefore, as long as the input operates in normal com- offset adjust range is asymmetrical, typically – 2mV to mon mode range, there will be very little common mode 8mV. At room temperature the input offset voltage of the induced distortion. Crossover distortion will increase as LT1635 is within the null range, thus the offset voltage can the output load resistance decreases. For the lowest dis- be adjusted to zero. Figure 1 shows the standard offset tortion, the LT1635 should be operated with the output adjustment. always sourcing current. V +
Reference
2 – 4 6 The reference of the LT1635 consists of a 200mV precision LT1635 3 + bandgap and a reference amplifier. As shown in the block 8 1 diagram, the 0.2V precision bandgap is referred to V – and 5 VREF is internally connected to the noninverting input of the ref- R1 erence amplifier. This configuration offers great flexibility 10k in that the reference voltage can be amplified or the reference V – 1635 F01 amplifier can be used as a comparator. Unlike the op amp, the output of the reference amplifier can only swing within
Figure 1. Standard Offset Adjustment
0.8V (typ) of the positive rail. To guarantee that the reference
Output
amplifier does not saturate over the industrial temperature The output voltage swing of the LT1635 is a function of range, the minimum operating supply should be 1.3V. The input overdrive as shown in the typical performance curves. reference amplifier can source 2mA of load current and can When monitoring voltages within 15mV of either rail, gain sink 10µA over the industrial temperature range. 9