Datasheet LT1187 (Analog Devices) - 9

LT1187
APPLICATIONS INFORMATION
The primary use of the LT1187 is in converting high speed
Power Supply Bypassing
differential signals to a single-ended output. The LT1187 The LT1187 is quite tolerant of power supply bypass- video difference amplifier has two uncommitted high ing. In some applications a 0.1µF ceramic disc capacitor input impedance (+) and (–) inputs. The amplifier has placed 1/2 inch from the amplifier is all that is required. In another set of inputs which can be used for reference and applications requiring good settling time, it is important feedback. Additionally, this set of inputs give gain adjust to use multiple bypass capacitors. A 0.1µF ceramic disc and DC control to the difference amplifier. The voltage in parallel with a 4.7µF tantalum is recommended. gain of the LT1187 is set like a conventional operational amplifier. Feedback is applied to Pin 8, and it is optimized
Calculating the Output Offset Voltage
for gains of 2 or greater. The amplifier can be operated single-ended by connecting either the (+) or (–) inputs to Both input stages contribute to the output offset voltage at the +/REF (Pin 1). The voltage gain is set by the resistors: Pin 6. The feedback correction forces balance in the input (R stages by introducing an input V FB + RG)/RG. OS at Pin 8. The complete expression for the output offset voltage is: Like the single-ended case, the differential voltage gain is set by the external resistors: (R V FB + RG)/RG. The maxi- OUT = (VOS + IOS(RS) + IB(RREF)) • (RFB + RG)/RG + IB(RFB) mum input differential signal for which the output wil RS represents the input source resistance, typically 75Ω, respond is approximately ±0.38V. and RREF represents the finite source impedance from the DC reference voltage, for VREF grounded, RREF = 0Ω. The S/D S/D IOS is normally a smal contributor and the expression V+ V+ 5 5 simplifies to: 3 V 7 3 IN + 7 + 2 – 2 V LT1187 6 IN – LT1187 6 VOUT = VOS(RFB + RG)/RG + IB(RFB) 1 OUT V 1 OUT V +/REF +/REF 8 8 –/FB –/FB If R 4 4 FB is limited to 1k the last term of the equation con- tributes only 2mV, since I V– V– B is less than 2µA. RFB RFB R RG R R R G G A FB + FB + V = + RG AV = – RG RG 7 V+ 6 S/D S/D V+ V+ 5 5 3 7 + 3 7 V + R IN DIFF 2 – VIN DIFF 2 FB LT1187 6 – LT1187 6 Q1 Q2 Q3 Q4 8 1 OUT V 1 IN V +/REF +/REF OUT V 8 RG 8 –/FB RG 4 VIN –/FB 4 3 + 2 – R + 1 REF E RE 1.1k 1.1k V– V– RS RS RREF RFB RFB 345µA 350µA RFB + RG R RG R V R O = (VIN DIFF + VIN) V FB + FB O = G ( – 4 V– R ( VIN DIFF ( R ( VIN RG G G 1187 F02 1187 F01
Figure 1. Figure 2. Simplified Input Stage Schematic
1187fb For more information www.linear.com/LT1187 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information ±5V Electrical Characteristics 5V Electrical Characteristics Typical Performance Characteristics Applications Information Simplified Schematic Package Description Revision History Related Parts